This adds a new data source for the Watcher decision engine that
implements the watcher.decision_engine.datasources.DataSourceBase.
related spec was merged at [1].
Implements: blueprint prometheus-datasource
[1] https://review.opendev.org/c/openstack/watcher-specs/+/933300
Change-Id: I6a70c4acc70a864c418cf347f5f6951cb92ec906
