Revert "Revert "Update jenkins jobs to optionally allow trusty""

This reverts commit 607b2f2ab4962dec4f4400ecc4687db2b206003a.

Jenkins masters have all had their gearman plugin updating which makes
them register jobs properly.

Conflicts:
	modules/openstack_project/files/zuul/layout.yaml

Change-Id: I3edeb26c9a1d3878b80266ea32d333bd8fc2f67e
Merge "Revert "Revert "Update jenkins jobs to optionally allow trusty"""