Control when ceph packages get updated

Control when ceph packages get updated

Currently, all the ceph package installation resources use
"state=latest", which means subsequent runs of the ceph playbooks
could result in ceph being upgraded if there are package updates
available in the selected repo.

This commit adds a new variable to ceph-common called
'upgrade_ceph_packages' which defaults to False.  This variable is used
in the package installation resources for ceph packages to determine if
the resource should use "state=present" or "state=latest".  If the
variable gets set to True, "state=latest" will be used.

Additionally, we update rolling_update.yml to override
upgrade_ceph_packages to true to permit package upgrades in this
context specifically.

Closes issue #506