When repository services are available, use them when creating a new repository

Summary:
Ref T2783. When creating a new repository, test for cluster services. If cluster services are available, allocate on a random open service.

Show the service that repositories are allocated on.

Test Plan: Created a new repository, saw it allocate onto an available cluster service.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2783

Differential Revision: https://secure.phabricator.com/D11003