[cern] Fix DCOS

* DCOS and mesos have the same validation methods but
 in out cherry-pick it was skipped.
* drop all neutron objects, replace _ with - in nova nodes, add
  cern-services false to instance metadata, replace first_address with
  resources.0.dcos_master_external_ip in dcoscluster apiaddress output.
* Add missing __init__.py
* Add missing monitor.py

Related-Ticket: OS-5206

[1] https://review.openstack.org/#/c/475682/8/magnum/api/validation.py@213

Change-Id: Id2e58163850e92d6959e19d3c48cb77c333b6d2f