akanda-horizon's hard-coded API address is not multi-RUG friendly

[Adam Gandelman]

The RUG currently exposes a REST API that horizon can use to issue the rug-ctl commands (upgrade, rebuild, etc) If we start to deploy multiple RUG instances, each exposes a REST API that can be used by horizon. Howver, currently akanda-horizon hard-codes the API address in local_settings.py.

We should instead start to publish a rug-ctl API endpoint in keystone, and have akanda-horizon look it up there like it does for all other service endpoints.

This will provide consistency for opreators. They will be free to put RUG APIs behind a webserver/balancer to provide availablity/load distribution, in the same way they would for the other services deployed.