Provide multiple git mount points

TODOS:

Phase 1:

Gitlab CE

MRs: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4657/builds https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4578

• Implement new configuration keys on gitlab.yml
• Add new db attribute to projects to point to their shard (default: null shard)
• Add new repo path lookup helpers and point exsting code to use it
• Monkey patch Gitlab.config.gitlab_shell.repos_path to throw an exception
• Update gitlab-shell calls to account for the shards
• Setup api/v3/internals for gitlab-shell to get the shard paths for a project

Gitlab Shell

MR: gitlab-org/gitlab-shell!61 (merged)

• Remove repos paths from config.yml (?)
• Update gitlab-shell bins (check, create_hooks, install) (ask /api/v3/internal for the paths?)
• Refactor GitlabProjects to account for the shards
• Get shards from api/v3/internals (when gitlab-shell gets called from git over ssh, for example in hooks and when verifying keys)

Gitlab Omnibus

• Update templates and cookbooks (see gitlab-org/omnibus-gitlab#1362 (closed))

Phase 2:

Gitlab EE

• Add tooling to move projects between shards

cc/ @ayufan @jnijhof @northrup @jacobvosmaer-gitlab @DouweM @pcarranza @pacoguzman