MetricsWeigher: Added support of unavailable metrics

[OpenStack Infra]

https://review.openstack.org/62813
commit 717f849b60b6267b19861348a20b2ebbc0db994d
Author: Lianhao Lu <email address hidden>
Date: Wed Dec 18 14:15:35 2013 +0800

    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

[Summer Long (slong-g)]

This was added into the Config Ref 'Weights' section in: https://review.openstack.org/#/c/87446
Also already in the automatically generated table:
Table 2.48. Description of configuration options for scheduling

Closing currentRelease.