Extend manage-projects exec timeout.

* modules/openstack_project/manifests/gerrit.pp: Change the
manage-projects exec timeout from the default 5 minutes to 15.

Change-Id: I47511f3e0c6d567edea0c65e0a9549fd3c798956
Reviewed-on: https://review.openstack.org/26171
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Paul Belanger <paul.belanger@polybeacon.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins