Wrap AuthorizedKeysCommand in Match statement

What does this MR do?

This change wraps the AuthorizedKeysCommand and AuthorizedKeysCommandUser values for sshd_config in a Match statement that limits them to the git user.

Some customers will have to do this because they have existing an AuthorizedKeysCommand, typically as an additional authentication step. By adding the Match we allow them to use their existing command and GitLab's command at the same time.