Issue #12 - Support ssh gateways.

[rteabeault]

Looking for feedback...

This commit provides support for connecting to the destination host via an SSH gateway.

• connector_port_open? implementation for winrm and ssh
• the ssh implementation checks if it can connect to the gateway if the gateway is given. If
  it is not given then connect directly to the destination.
• host_commander#connector_for now calls the connector_port_open? for winrm and then ssh.
• port_open? moved to host_connector so it could be called from the winrm and ssh host connectors.
• ssh connector's run method now creates a gateway connection if the gateway value is in the :ssh options.

[KAllan357]

I think I'm okay with the format of user@host:port, but its a bit different than what might be expected. I think it might be worth having some error checking here to make sure the variables are split appropriately.

[rteabeault]

Would you rather the config be something like

ssh: {
  gateway: {
    user: "foo",
    host: "bar.com",
    port: 1234
  }
}

I picked the format user@host:port because it is the format that knife ssh uses. We can add some error checking but because of the possible characters allowed in a user and host name it may not be 100% correct.

[rteabeault]

@KAllan357, any thoughts on my previous comment?

[jefmsmit]

I personally like the user@host:port better because it conforms to the knife ssh style. Any other thoughts?

[jefmsmit]

Whoops, should have added @KAllan357 and @rteabeault since they're the most familiar.

[capoferro]

@KAllan357 @reset @ivey This seems reasonable to me. We've unfortunately let this PR languish. I will spend some time tomorrow to see if this can be rebased easily.

Any thoughts on why this might not be ready for merge pending a rebase?

[capoferro]

Rebased on master in #26

[capoferro]

Closing this one in favor of #26 (same commit, rebased)