ceph client packages

Unfortunately, glance::backend::rbd already includes one ceph client
package dependency, 'python-ceph'.  So, we tap dance around whether
glance::backend::rbd itself has been declared and thus declares
'python-ceph'.
I wanted to write we should have str2bool_i here, but i see that in pacemaker::glance and others we compare against string too, so we'd better be consistent for now i think. However we just compare `== 'true'` so far, grep for `== 'false'` shows no results, so maybe it would be safer to have

    if (map_params('include_glance') == 'true') {
    }
    else {
    }

here, so that we don't introduce a "third option" (e.g. for "False") where none of the two branches would get evaluated.