Fluentd not starting if backend not reachable

Perhaps this is connected to https://github.com/fluent/fluentd/issues/1280 and it seems like a general mistake in design.
A failed backend connection definitely must not prevent fluentd from starting at all.

```
/usr/lib/ruby/2.5.0/net/http.rb:939:in `rescue in block in connect': Failed to open TCP connection to couch:5984 (getaddrinfo: Try again) (SocketError)
from /usr/lib/ruby/2.5.0/net/http.rb:936:in `block in connect'
```
I am using the official Docker image `fluent/fluentd:latest`

Fluentd is widely used to fire-and-forget messages, often without really caring if some messages get lost.
It is a complete show-stopper if it refuses to start when backends are not available.

There should be a configuration switch to silently ignore outages and just warn on start, at least.