Fix monasca_checks host_alive YAML configuration

The YAML structure for monasca_checks' host_alive was incorrect in the
playbooks, preventing host_alive checks from functioning.  One would
see no host_alive_status metrics, and this error in collector.log:

.host_alive(services_checks.py:92) | Each service check must have a name

Change-Id: I52a91362d7c958d27087933ccd91de326da9392e