In change I22234e0bb6b3a1cecb29a6b99a3afcd02ffdbf5f added a
get_registered_limits() interface but missed making it work with the
Enforcer cache because caching was implemented in a higher layer,
_get_limit() and get_registered_limits() could not make use of
_get_limit().
This moves the caching logic to where limits are retrieved from
keystone, so that get_registered_limits() will use caching and any
interfaces that might be added in the future will be able to benefit
from using the cache.
Closes-Bug: #1964848
Change-Id: I28cdd4d3f4927b94acea3309b36999850cf2ee2a
c67c9cf0df