memcached and nova-cc relation sets up incorrect ufw rules in multi-network env

Not sure where is the upstream branch of memcached charm, but the revision in charm store relies on relation_get('private-address') to get an IP address of nova-cloud-controller unit.

That ends up with unexpected ufw rules in multi-network environment. For example, if memcached only has 192.168.0.0/24 network and nova-cc has 192.168.0.0/24 and 10.0.0.0/24 networks. An IP adddress from 10.0.0.0/24 will be chosen as unit private address and will be written, but nova-cc will access to memcached from 192.168.0.0/24. So no allowed rules in ufw will match.

It would be nice if memcached relation supported network bindings to detect the correct IP addresses for ufw rules reliably.

# ufw status verbose
Status: active
Logging: on (low)
Default: allow (incoming), allow (outgoing), deny (routed)
New profiles: skip

To Action From
-- ------ ----
11211/tcp ALLOW IN 10.0.0.XY
22 ALLOW IN Anywhere
11211/tcp DENY IN Anywhere
22 (v6) ALLOW IN Anywhere (v6)
11211/tcp (v6) DENY IN Anywhere (v6)

[hooks/memcached_hooks.py]
@hooks.hook('cache-relation-joined')
def cache_relation_joined():

    settings = {'host': unit_get('private-address'),
                'port': config('tcp-port'),
                'udp-port': config('udp-port')}

    for rid in relation_ids('cache'):
        relation_set(rid, **settings)

    addr = relation_get('private-address')
    if addr:
        log('Granting memcached access to {}'.format(addr), level='INFO')
        memcached_utils.grant_access(addr)