Wait for groups to be created before getting uuids

* jeepyb/cmd/manage_projects.py: When it is necessary to create a group
in gerrit, check that the group appears in ls-groups before getting the
group UUID from the database. Otherwise there is a race with
create-group and updating the databse that can result in the UUID not
being present in the DB.

Change-Id: I7733e5550e493ec72d9afe022f1d1426d3f15f83
Merge "Wait for groups to be created before getting uuids"