Add missing Pacemaker constraint against Keystone

Add missing Pacemaker constraint against Keystone

The Heat contraints group was missing the initial
dependency on Keystone, causing Pacemaker to Heat before or
in parallel to Keystone.

Given Systemd can define dependencies in the unit files, this was
additionally causing an unmanaged start of Keystone making
cluster initialization to fail (with Keystone start timeout blocking
all the depending resources).

Also moves Keystone -> Ceilomter constraint on top of Ceilometer
constraints group for clarity.

Logs and more infos at [1]

1. https://bugzilla.redhat.com/show_bug.cgi?id=1235703

Change-Id: I9505fd46c5bf278afc8ff919c7e768e2de194cb8