Drop the ref from the tag fetch in GGP

If you start with an out of date repo (we do -- we clone from cached
git repos on our images) and the git remote update fails (eg due to
mirror sync issues) and you then do a fetch with a ref argument
(eg refs/tags/foo), it will fail with:

  error: no such remote ref refs/tags/foo

However, git fetch --tags will get all the tags from the zuul merger,
which is what we really care about, and the subsequent checkout and
reset will make sure that we really did get the tag we wanted.
Therefore, remove the ref argument that can cause these occasional
failures.

Change-Id: Iaed8b82204a3eddc7ee418ebe94637f2a371d25b