Added a new scheduler metrics weight plugin

The new metrics weigher can compute the weight based on the compute
node host's metrics data. The to-be weighed metrics and their
weighing ratio are specified in the configuration file as the
followings:

    metrics_weight_setting = name1=1.0,name2=-1.0

The final weight would be name1.value * 1.0 + name2.value * (-1.0).

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

DocImpact

Change-Id: Ib3e68505e6d4d8f6d67b54c5f00de3e1c172738c
Merge "Added a new scheduler metrics weight plugin"