Get allowed hosts ignores prefix

The current implementation of get_allowed_hosts [0] is prefix unaware and assumes the database name as the prefix:

            # First check for setting with prefix, then without
            for attr in ["%s_hostname" % (database), 'hostname']:
                hosts = settings.get(attr, None)
                if hosts:
                    break

This can lead to missing allowed_hosts if the prefix is not the database name and the address is different than private-address. i.e. potentially in IPv6 scenarios.

TRIAGE:
Make get_allowed_hosts prefix aware.
Bonus: Make mysql-router set private-address to match.
Bonus bonus: Make mysql-router send *all* potential addresses that the client might use. This can be a json encoded list.

[0] https://github.com/juju/charm-helpers/blob/master/charmhelpers/contrib/database/mysql.py#L431