Replace the use of "unit.public_address" with
zaza.model.get_unit_public_address().
AssertionError: {'host': '172.16.0.25', 'port': 3306, 'user': 'prom_exporter'} != {'host': None, 'port': 3306, 'user': 'prom_exporter'}
- {'host': '172.16.0.25', 'port': 3306, 'user': 'prom_exporter'}
? ^^^^^^^^^^^^^
+ {'host': None, 'port': 3306, 'user': 'prom_exporter'}
Related-Bug: openstack-charmers/zaza#472