ceph::mon create or destroy monitors

* ceph::mon resource

  create a MON if ensure == present or destroy one if absent

  all combinations of keys / keyring as specified in the blueprint are
  supported

* integration tests cover all cases and unit tests are implemented to
  ensure syntax correctness

* minor corrections to other classes are included ( two nodes for
  integration tests, default values for the ceph class etc. )

* A ceph::mons class wrapper is created for compatibility with
  scenario_node_terminus

Closes-Bug: #1251443

Change-Id: Ic56a4124738006f7946ee9c166295f7048cb7854
Signed-off-by: Loic Dachary <loic@dachary.org>