Gitea backend: add option to use SSH when fetching repositories

[MagicRB]

GItea/Forgejo allow the administrator to disable HTTP Git, I did that on my instance but for now buildbot-nix insists on using HTTP. The fix is quite simple I'll write the code tomorrow, this exists so I don't forget.

As for the actual option in GItea/Forgejo that does is: `repository.DISABLE_HTTP_GIT``. As for the security benefit of this disabling ¯_(ツ)_/¯ but it seemed like a good idea since I never use HTTP git anyway.