better support for rbd in glance recipes

- setting node['ceph']['install_method'] to 'chef' will install all ceph
  packages, drop the relevant config, and create the correct user/pool
  for glance to use. This attribute was necessary to allow a manually
  installed ceph cluster to also be used if desired (that is, if set
  to 'manual', or some other value than 'chef', only the glance
  api.conf will be configured and no actual ceph setup will be
  attempted)
- added an attribute to control pg_num when creating rbd pool