Flavor-specific instance quotas

Support per-instance-type quota resources so that for each existing instance-type, there is the potential to add a new quota limiting *only* that instance type (and keep the existing instances quotas as an over-arching limit).

For example, if the following quotas were set:

  instances: 110
  instances-m1.xlarge: 10
  instances-m1.large: 20
  instances-m1.small: 50
  instances-m1.tiny: 100

and a user requested an additional xlarge instance, we'd first check if we had headroom on the instances-m1.xlarge quota and then if we also had headroom on the over-arching instances quota (before going on to check the ram & cores
if necessary). Whereas, if a medium instance was requested, we would only check the overarching limit, as there is no instances-medium quota defined.

This would require some change to the quotas logic, to allow the set of resources that may be limited by quotas to be more dynamic (currently we have a fairly fixed set, whereas new instance types may be defined at any time).

The requirement was raised on this ML thread:

  https://lists.launchpad.net/openstack/msg14925.html