Added a new scheduler filter for metrics

The scheduler filter MetricsFilter filters out those hosts which don't
have the metrics data available as required by metric settings.

This is part of the blueprint utilization-aware-scheduling.

DocImpact: Added a new metrics filter.

Change-Id: Ib4a898774daf683c4496ef3e9953d23027f11ac0
Merge "Added a new scheduler filter for metrics"