SSH push mirroring support

Following these two issues:

• https://gitlab.com/gitlab-org/gitlab-ee/issues/2954
• https://gitlab.com/gitlab-org/gitlab-ee/issues/3109

we can add support for SSH to push mirroring, in the same way as was done for pull mirroring in https://gitlab.com/gitlab-org/gitlab-ee/issues/98

mentioned in issue #98 (closed)

SSH probably works at the moment with the git user's SSH configuration, but this is not supported and of no use for public installations like GitLab.com, anyway.

added EE Starter label

mentioned in issue gitlab-com/support-forum#2428 (closed)