Investigate endpoint, service, and region configuration options

The current implementation of oslo.limit requires that services know who they are. To do this, operators need to set a configuration option that tell each service which service they are and what region they are in.

Currently, this is achieved using endpoint_ids, since endpoints reference both services and regions. Oslo.limit and query keystone for information about the service and region based on the endpoint ID in configuration. Endpoint IDs aren't as user friendly as service name + region name configuration options.

We should ask operators which approach is better, since they're the ones that have to configure the system. Based on their answer, we should either implement new configuration options for services and region or keep the single endpoint ID option we have today.

This was discussed during a group code review of the oslo.limit implementation [0].

[0] http://eavesdrop.openstack.org/irclogs/%23openstack-keystone/%23openstack-keystone.2019-07-02.log.html#t2019-07-02T16:32:50