refactor Ceph nova_compute.pp

 * declare ceph::nova_compute only for compute role, it's useless in
   ceph::controller
 * drop TODO for moving the RBD secret.xml to mktemp (it's much safer to
   keep it in /root/, and virsh is unable to accept it on stdin so we
   can't get rid of it altogether)
 * move external relationship declarations to init.pp
 * drop nova-compute service redefinition, include nova::compute from
   init.pp instead