VMware: add resource limits for CPU

The flavor extra specs for quotas has been extended to support
 quota:cpu_limit - upper bound of CPU allocation (MHz)
 quota:cpu_reservation -  guaranteed minimum CPU allocation (MHz)

This enables the VMware driver to provide enable CPU allocations
for the running instance.

DocImpact

Change-Id: I1bec4f009285f7991eaaa8155329366a4eba5e1f
Merge "VMware: add resource limits for CPU"