When creating a repository with EditEngine, allocate it onto a random cluster service

Summary: Ref T10748. This copies existing code in the CreateController which will eventually be removed.

Test Plan:

• Created a new repository with the EditPro workflow.
• Saw it come up into the cluster properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10748

Differential Revision: https://secure.phabricator.com/D15835