Support deployment of gitlab-sshd (omnibus)

gitlab-sshd is a new binary generated by gitlab-shell, currently in alpha. It's a dedicated, persistent SSH server that can be used instead of / in addition to integrating with OpenSSH.

Upsides are improved resource use per SSH connection and simpler deployment; the main downside is that it requires a dedicated SSH IP/port.

The main use case is containerized deployments of gitlab, but we may want to include it in omnibus as well.