Add $package_ensure to glance::backend::rbd

Previously for the python-ceph package ensure was hard coded to 'present', but
in some circumstances we want to set the ensure value same as for the glance
package itself.  This adds that in.

Change-Id: Ib5fc0b3d916eef93c810b0ee3fcecf0d9b80218e
Merge "Add $package_ensure to glance::backend::rbd"