Update opnfv-docs Merge Job Trigger and Parameter

Update opnfv-docs Merge Job Trigger and Parameter

- Adds 'GERRIT_REFSPEC' parameter
   By setting a default to 'refs/heads/{branch}' (which will evaluate to
   either 'refs/heads/master', or 'refs/heads/stable/colorado') we will
   be able to manually trigger patchsets.

- Adds 'rebuild docs' comment to trigger
   Because 'remerge' is used by multiple projects, adding 'rebuild docs'
   will allow anyone to trigger only a rebuilding of the documentation,
   while still rebuilding them on a 'remerge' comment.

Change-Id: I596f3e8bf412077e5e505555cb0654394a32d213
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>