Fix keystone oidc_memcache_servers for IPv6 and add tests

In IPv6 clusters, the OIDC_MEMCACHE_SERVERS variable for Keystone was rendered in the format "[hostname]:port" instead of "hostname:port". As this isn't a valid format, keystone crash-looped with "failed to parse cache server".

This commit does a regex replace on the "inet6:[hostname]:port" structure returned by _generate_memcached_addresses.

( https://gitlab.com/yaook/operator/-/blob/96fe6da646fbfdc8354b189d0cd70c22e8ad2ed9/yaook/statemachine/cue.py#L718 )