Make CephConfigOverrides append to ceph.conf[global]
Make CephConfigOverrides append to ceph.conf[global]

Previously it was mistakenly replacing the contents because we
do not do deep merge.

Change-Id: I145feb0208f135da7c71694ebcecd937244d66b1
Closes-Bug: #1719919

openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates