Add config option ssh_options

This option allows to pass all ssh options to the tunnel.

I might need this with `ssh_options: StrictHostKeyChecking=no` in my [chef cookbook for beaver](https://github.com/alappe/cookbook-beaver).

It's written so that nodes can publish their public key to chef where the logstash master adds them to an `authorized_keys` file. But unless I manually accept the fingerprint, they cannot connect. With this option above I get around this issue with giving up some of the security… I'm not sure yet but I think having the option in beaver doesn't hurt and gives flexibility? Do you think that's useful?

Note: I didn't simply add BatchMode=yes to the ssh_options default because then people would probably accidentally overwrite it…

Note2: Please feel free to write it differently, it's been the first python since about a year…