Add Gerrit Trigger on Tag Creation

Add Gerrit Trigger on Tag Creation

We should be able to capture a tag creation event by triggering on
'ref-updated' but looking specifically at the 'refs/tags/' branch
prefix.

Though this will trigger a build against a specific tag, scripts will
still need to examine the GERRIT_REFSPEC to determine the tag name.

Change-Id: Ic18a3e11530fb71735431240f93d2074dacf4e1c
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>