refactor Ceph init.pp

 * extract ceph.conf handling into ceph::conf
 * rename ceph::deploy to ceph::mds (and leave it disabled)
 * move libnss handling to ceph::libnss class in params.pp
 * move ceph service definition to ceph::params
 * simplify role-dependent declarations and ordering rules
 * include ceph::cinder for cinder nodes, but don't include ceph classes
   by default for unrecognized node roles
 * install python-pushy instead of pushy in Debian-based systems
 * minor formatting fixes to calm down puppet-lint