Add CephConfigOverrides to allow arbitrary configs in ceph.conf

Add CephConfigOverrides to allow arbitrary configs in ceph.conf

We need to reuse the ceph_conf_overrides structure provided by
ceph-ansible for both user provided configs and TripleO managed
configs. This change merges the special user facing parameter
with the TripleO generated configs.

Also adds osd_scenario and osd_objectstore params for compatibility
with newer ceph-ansible versions.

Change-Id: I29c689c6c689590da5b6a3f581fdbec98a52e207
Closes-Bug: #1715321
(cherry picked from commit 32bc2abf14af4ca1449e18b848e2be3cff013987)