[RFE] Remove the manager.py indirection layer

[Pedro Castillo]

The manager.py indirection layer acts as a type of proxy for keystone that receives JSON-formatted RPC calls through a UNIX socket and forwards them to the keystone server. This layer was added when there was a Python version mismatch between the running charm operator code and the keystone client code.

Since this version mismatch is not relevant anymore given that both keystone and the charm SDK are fully compatible with Python 3, the manager.py indirection layer can be removed and the calls to keystone can be done directly from the charm code without having to spawn a subprocess.

Removing this layer of indirection will make the code a lot more simple to reason about, and would also remove some code that is hard to reason about (see `KeystoneManagerProxy` in `hooks/keystone_utils.py` line 1218).

[Corey Bryant (corey.bryant)]

Thank you Pedro. This makes sense. I've subscribed Alex as he developed this and would be the best person to triage this.