Add the concept of resource reservation in neutron.
Usage tracking logic is also updated to support reservations.
Reservations are not however available with the now deprecated
configuration-based quota driver.
The base API controller will now use reservations to perform
quota checks rather than counting resource usage and then
invoking the limit_check routine.
The limit_check routine however has not been removed and
depreacated as a part of this patch. In order to ensure all
quota drivers expose a consistent interface, a
make_reservation method has been added to the configuration
based driver as well. This method simply performs "old-style"
limit checks by counting resource usage and then invoking
limit_check.
DocImpact
Implements blueprint better-quotas.
Change-Id: Ifea07f461def564884af5b291c8a56655a4d818b
574b25b857