MetricsWeigher: Added support of unavailable metrics

This patch added the support of unavailable metrics in the metric
weigher. Based on the configuration, if an unavailable metric is met,
the metric weigher might either throw an exception, or return an
configurable weight value.

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

DocImpact: Added support of unavailable metrics. 2 new config options
are added, 1) 'required' is a boolean option indicating whether all
the metrics set by 'weight_setting' are needed to be available when
calculating the weight. 2) 'weight_of_unavailable' is a float option
specifying the value to be returned if the metrics is unavailable. See
etc/nova/nova.conf.sample for details.

Change-Id: Icb951605aae28487b00a12d172a3ac4dafda2cdb
Merge "MetricsWeigher: Added support of unavailable metrics"