Strategies can require, from Watcher, metrics collected on the IAAS.
Watcher uses only Ceilometer API to retrieve metrics.
Change the metrics database backend, add a new metric is not in
the scope of Watcher, but rather in the Ceilometer one. We add some
links to Ceilometer documentation about these topics.
Change-Id: If37c7df8e5852f5ecf94b4a9eb9c8c91fe6637eb
894dfa0d7e