Document the Zuul triggers

The zuul and zuul-post had sparse documentation, improve it a bit and
point to upstream documentation at:
http://ci.openstack.org/zuul/launchers.html#zuul-parameters

Add a note that Zuul 2.0 (gearman based) no longer requires those
parameters to be explicitly defined.

Change-Id: I1edcd5c3a101e2afe0025ad4cd3e511582998999
Merge "Document the Zuul triggers"