Wait for ACL creation in manage-projects

* jeepyb/cmd/manage_projects.py: After Gerrit creates
refs/meta/config, wait for the empty project.config file to get
committed before attempting to push the proposed ACL.

Change-Id: I8601f90bc7b93cecffdc77073ef51f445e7539b6
Merge "Wait for ACL creation in manage-projects"