mysql-innodb-cluster unit fails when grant does not exist for host

[Bas de Bruijne]

In test run https://solutions.qa.canonical.com/v2/testruns/48b95cac-f227-4474-8935-6d44d6bdc9e7, mysql-innodb-cluster fails with the error:

```
unit-mysql-innodb-cluster-2: 07:08:09 DEBUG unit.mysql-innodb-cluster/2.juju-log db-router:58: Grant does NOT exist for host '192.168.33.171' on db 'barbican'
unit-mysql-innodb-cluster-2: 07:08:09 ERROR unit.mysql-innodb-cluster/2.juju-log db-router:58: Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/.venv/lib/python3.10/site-packages/charms/reactive/__init__.py", line 74, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/.venv/lib/python3.10/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/.venv/lib/python3.10/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/.venv/lib/python3.10/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/charm/reactive/mysql_innodb_cluster_handlers.py", line 340, in db_router_respond
    if instance.create_databases_and_users(db_router):
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/charm/lib/charm/openstack/mysql_innodb_cluster.py", line 1309, in create_databases_and_users
    allowed_units = self.get_allowed_units(
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/charm/lib/charm/openstack/mysql_innodb_cluster.py", line 1262, in get_allowed_units
    allowed_units = db_helper.get_allowed_units(
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/.venv/lib/python3.10/site-packages/charmhelpers/contrib/database/mysql.py", line 456, in get_allowed_units
    hosts = [settings['private-address']]
KeyError: 'private-address'
```

The source problem is a juju bug (LP: #1996218) but this should not put the cluster into an error state.

Logs and crashdumps can be found here:
https://oil-jenkins.canonical.com/artifacts/48b95cac-f227-4474-8935-6d44d6bdc9e7/index.html

[Felipe Reyes (freyes)]

Based on https://bugs.launchpad.net/juju/+bug/1996218/comments/1 , I'm marking this as triaged and high, since a missing "private-address" is part of "data not ready yet" state and the charm needs to handle this gracefully.