requires: Add interface method to retrieve all URLs

[javacruft]

In a clustered etcd deployment, clients may be configured with a full list of remote etcd units for communication.

Add a helper to the interface to retrieve this information.

[Cynerva]

I'm pretty sure get_connection_string from any one unit already returns the URLs for all ETCD units, separated by commas. They communicate their addresses to each other over the cluster (peers) relation.

See:

• peers.py:get_db_ingress_addresses
• layer-etcd/reactive/etcd.py:send_cluster_connection_details
• layer-etcd/lib/etcdctl.py:get_connection_string

[javacruft]

OK I'd missed that bit