Enable OpenSSH server on FreeBSD nodes

Summary:
When installing a FreeBSD server from an image (.vmdk disk for example),
there is no opportunity to say "enable sshd" at install time.

As such, this responsibility belongs to Salt to configure it.

Test Plan: Deploy on thrayce

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3090