Add support for instance CPU consumption control.

Part 4 for blueprint quota-instance-resource, by setting cpu_shares,
cpu_quota,cpu_period through nova flavor-set with admin credentials,
 the cloud admin can implement flexable policy on VM vcpu to physical
CPU consumption ratio.

for example:
nova flavor-key m1.low_cpu set cpu_quota=10000
nova flavor-key m1.low_cpu set cpu_period=20000
the instance of m1.low_cpu can only has a max of 50% CPU consumption
of a physical CPU computing capability.

original reviews https://review.openstack.org/#/c/14151/
DocImpact
partly implement blueprint quota-instance-resource.

Change-Id: I9ea1229ee78a4d706b1a18c0f4f15157611952e4
Merge "Add support for instance CPU consumption control."