Organize limits units and per-units constants
Organize limits units and per-units constants

During v3 api review there was an agreement that would be nice to move limits
constants to utils file and refactor its usage inside limits api to avoid
unnecessary code duplication.

Also changed TestLimiter class to MockLimiter since that name can be confuse to
some unit test frameworks.

Change-Id: I1b8626e8d1e3257333d4dfee61591fd4cde82bd1

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects