blank access-network due to call to relation-set with blank value,

Hi

we need to file a bug on this: https://github.com/openstack/charm-percona-cluster/blob/master/hooks/percona_hooks.py#L811;

We have ran into a severe outage when shared-db-relation-changed function bombs on blank access-network due to call to relation-set with blank value, also, that should totally be deferring to spaces, too

the fix which was -- touch /tmp/seeded && chown mysql.mysql /tmp/seeded && mv /tmp/seeded /var/lib/mysql/seeded || and juju config mysql access-network="$IP" <-- see https://github.com/openstack/charm-percona-cluster/blob/stable/17.08/hooks/percona_hooks.py#L545

notes state that when config-change ran, line 202, it kicked to mark seeded in the charm state - the only other time seeded gets hooked up is if you upgrade-charm; which is why it was believed that 'access-network' value was needed due to be set.

E.g:

$ juju run --unit mysql/0 hooks/shared-db-relation-changed
ERROR no relation id specified
ERROR no relation id specified
Traceback (most recent call last):
  File "hooks/shared-db-relation-changed", line 709, in <module>
    main()
  File "hooks/shared-db-relation-changed", line 701, in main
    hooks.execute(sys.argv)
  File "/var/lib/juju/agents/unit-mysql-0/charm/hooks/charmhelpers/core/hookenv.py", line 731, in execute
    self._hooks[hook_name]()
  File "hooks/shared-db-relation-changed", line 493, in shared_db_changed
    relation_settings={'access-network': access_network})
  File "/var/lib/juju/agents/unit-mysql-0/charm/hooks/charmhelpers/contrib/peerstorage/__init__.py", line 256, in peer_store_and_set
    **kwargs)
  File "/var/lib/juju/agents/unit-mysql-0/charm/hooks/charmhelpers/contrib/peerstorage/__init__.py", line 140, in relation_set
    relation_settings=relation_settings, **kwargs)
  File "/var/lib/juju/agents/unit-mysql-0/charm/hooks/charmhelpers/core/hookenv.py", line 391, in relation_set
    relation_cmd_line + ["--file", settings_file.name])
  File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['relation-set', '--file', '/tmp/tmpuLehYo']' returned non-zero exit status 2

As such it looked like there's definitely a bug there that you can't relation-set a null value; this being run blindly is not gonna work https://github.com/openstack/charm-percona-cluster/blob/stable/17.08/hooks/percona_hooks.py#L548