rework the charm to the new design

[lucabello]

Enhancement Proposal

The coordinator charm has to:

1. instead of supporting one relation endpoint per role, we should relate the coordinator to the worker over the single interface/endpoint mimir-cluster;
2. gather all the roles from relation data and check if the minimal subset conditions are met; if not, set BlockedStatus and let the user know with a status message;
3. build the Mimir configuration and update it with the hash ring (memeberlist), composed of all the URLs of the workers (through the top-level config key memberlist, under join_members);
4. send the Mimir configuration to all the workers over relation data

Completing this issue should be enough for a functioning testable charm to be related to a worker.