Support gitlab-sshd (in addition to openssh) for git-over-ssh functionality

Overview

gitlab-shell!394 (merged) looks to implement a built-in SSH daemon for gitlab-shell. The GDK currently supports running the OpenSSH daemon but is disabled by default. A built-in SSH daemon brings a lot of benefits, with the most relevant here being the removal of needing to install OpenSSH for GDK in order to access GitLab via SSH.

Proposal (optional)

Once gitlab-shell!394 (merged) is merged, we should make the default SSH daemon used in GDK to be the gitlab-shell built-in one. We should also add an option to gdk.yml to allow switching between the different daemon types.

Something like:

---
sshd:
  use_gitlab_sshd: true # or false for OpenSSH