Specify the bind address for SSH communicator tunnels

#### Description

SSH tunnels specified by `ssh_remote_tunnels` and `ssh_local_tunnels` can only bind to localhost. I'd like to propose that the tunnel parsing code be extended to allow for standard OpenSSH-style tunnel specifications: `[bind_address:]port:host:hostport`

This is mentioned in tunnel.go as NYI. Is it on the cards?

#### Use Case(s)

Specific use-case is for building images on remote ESXi servers, where floppy & CD preseeds are not possible for driver reasons. Having packer forward a port on ESXi server to packer's HTTP server saves having to setup a separate SSH session.

#### Potential configuration

```
"ssh_remote_tunnels": ["esxi.server.ip.address:55555:{{ .HTTPIP }}:{{ .HTTPPort }}"]
```