Use ipv6 address as default when out_forward and macOS catalina

**Describe the bug**

out_forward tries to connect `::1` instead of `127.0.0.1` because the following code uses first value(which is ipv6 address) as the hostname.

https://github.com/fluent/fluentd/blob/8f2ea4bb26e5735b36acf5f6dc0fa5796ce4c4f9/lib/fluent/plugin/out_forward.rb#L731

https://github.com/fluent/fluentd/issues/2865#issuecomment-596321456

**To Reproduce**

Needs macOS catalina (I'm not sure other versions of macOS)

server
```

@type forward

@type stdout

```

client
```

@type dummy
tag test

@type forward

host localhost // if it's 127.0.0.1 then it works well
port 24224

flush_interval 0

```

getaddrinfo(3) returns the values in a different order on ubuntu and macOS.

```sh
# ubuntu:bionic
$ docker run --rm -it rubylang/ruby:2.6.5-bionic ruby -e 'require "socket"; p Socket.getaddrinfo("localhost", 80, nil, Socket::SOCK_STREAM)'
[["AF_INET", 80, "127.0.0.1", "127.0.0.1", 2, 1, 6], ["AF_INET6", 80, "::1", "::1", 10, 1, 6]]
$ docker run --rm -it rubylang/ruby:2.7.0-bionic ruby -e 'require "socket"; p Socket.getaddrinfo("localhost", 80, nil, Socket::SOCK_STREAM)'
[["AF_INET", 80, "127.0.0.1", "127.0.0.1", 2, 1, 6], ["AF_INET6", 80, "::1", "::1", 10, 1, 6]]

# macos:catalina
$ rbenv local 2.6.5; bundle exec ruby -e 'require "socket"; p Socket.getaddrinfo("localhost", 80, nil, Socket::SOCK_STREAM)'
[["AF_INET6", 80, "::1", "::1", 30, 1, 6], ["AF_INET", 80, "127.0.0.1", "127.0.0.1", 2, 1, 6]]
$ rbenv local 2.7.0; bundle exec ruby -e 'require "socket"; p Socket.getaddrinfo("localhost", 80, nil, Socket::SOCK_STREAM)'
[["AF_INET6", 80, "::1", "::1", 30, 1, 6], ["AF_INET", 80, "127.0.0.1", "127.0.0.1", 2, 1, 6]]
```

**Expected behavior**

Use ipv6 address when setting `localhost` to host at server section of out_forward

**Your Environment**

- Fluentd or td-agent version: at least 1.8.0
- Operating system:

```
$ sw_vers
ProductName: Mac OS X
ProductVersion: 10.15.3
BuildVersion: 19D76
```

- Kernel version: 19.3.0

**Your Configuration**

Same as the To Reproduce

**Your Error Log**

```
2020-03-09 15:00:30 +0900 [info]: parsing config file is succeeded path="example/out_forward.conf"
2020-03-09 15:00:30 +0900 [info]: gem 'fluentd' version '1.8.0'
2020-03-09 15:00:30 +0900 [info]: 'flush_interval' is configured at out side of . 'flush_mode' is set to 'interval' to keep existing behaviour
2020-03-09 15:00:30 +0900 [info]: adding forwarding server 'localhost:24224' host="localhost" port=24224 weight=60 plugin_id="object:eec"
2020-03-09 15:00:30 +0900 [warn]: both of Plugin @id and path for are not specified. Using on-memory store.
2020-03-09 15:00:30 +0900 [warn]: both of Plugin @id and path for are not specified. Using on-memory store.
2020-03-09 15:00:30 +0900 [info]: using configuration file:

@type dummy
tag "test"


@type forward
flush_interval 0

host "localhost"
port 24224


flush_interval 0

2020-03-09 15:00:30 +0900 [info]: starting fluentd-1.8.0 pid=6735 ruby="2.7.0"
2020-03-09 15:00:30 +0900 [info]: spawn command to main: cmdline=["/Users/yuta.iwama/.rbenv/versions/2.7.0/bin/ruby", "-Eascii-8bit:ascii-8bit", "-r/Users/yuta.iwama/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/bundler-2.1.4/lib/bundler/setup", "/Users/yuta.iwama/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/bin/fluentd", "-c", "example/out_forward.conf", "--under-supervisor"]
/Users/yuta.iwama/src/github.com/fluent/fluentd/lib/fluent/supervisor.rb:796: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/Users/yuta.iwama/src/github.com/fluent/fluentd/lib/fluent/system_config.rb:132: warning: The called method `overwrite_variables' is defined here
2020-03-09 15:00:31 +0900 [info]: adding match pattern="test" type="forward"
2020-03-09 15:00:31 +0900 [info]: #0 'flush_interval' is configured at out side of . 'flush_mode' is set to 'interval' to keep existing behaviour
2020-03-09 15:00:31 +0900 [info]: #0 adding forwarding server 'localhost:24224' host="localhost" port=24224 weight=60 plugin_id="object:ed8"
2020-03-09 15:00:31 +0900 [info]: adding source type="dummy"
2020-03-09 15:00:31 +0900 [warn]: #0 both of Plugin @id and path for are not specified. Using on-memory store.
2020-03-09 15:00:31 +0900 [warn]: #0 both of Plugin @id and path for are not specified. Using on-memory store.
2020-03-09 15:00:31 +0900 [info]: #0 starting fluentd worker pid=6749 ppid=6735 worker=0
2020-03-09 15:00:31 +0900 [info]: #0 fluentd worker is now running worker=0
2020-03-09 15:00:33 +0900 [warn]: #0 failed to flush the buffer. retry_time=0 next_retry_seconds=2020-03-09 15:00:34.095101 +0900 chunk="5a065b88ecab18d92b81f21cee90a2c9" error_class=Errno::ECONNREFUSED error="Connection refused - connect(2) for \"::1\" port 24224"
```

**Additional context**

Start in lib/fluent/plugin/out_forward.rb around the linked line and reproduce the localhost resolution behavior with Socket.getaddrinfo on macOS Catalina and Ubuntu. Compare the selected address with the issue's expected behavior and verify the forwarding connection succeeds against the provided configuration; the issue's IPv4/IPv6 wording should be clarified before implementation.

Written by the indexing model from the issue text.