empty .juju-persistent-config breaks charm hooks

[Edward Hope-Morley]

If for whatever reason you have an empty .juju-persistent-config state file (normally contains values of previously set config opts calls to Config.config() will fail with e.g.

2018-04-27 14:32:14 INFO config-changed Traceback (most recent call last):
2018-04-27 14:32:14 INFO config-changed File "/var/lib/juju/agents/unit-hacluster-rados-1/charm/hooks/config-changed", line 481, in <module>
2018-04-27 14:32:14 INFO config-changed hooks.execute(sys.argv)
2018-04-27 14:32:14 INFO config-changed File "/var/lib/juju/agents/unit-hacluster-rados-1/charm/hooks/charmhelpers/core/hookenv.py", line 805, in execute
2018-04-27 14:32:14 INFO config-changed self._hooks[hook_name]()
2018-04-27 14:32:14 INFO config-changed File "/var/lib/juju/agents/unit-hacluster-rados-1/charm/hooks/config-changed", line 157, in config_changed
2018-04-27 14:32:14 INFO config-changed update_nrpe_config()
2018-04-27 14:32:14 INFO config-changed File "/var/lib/juju/agents/unit-hacluster-rados-1/charm/hooks/config-changed", line 450, in update_nrpe_config
2018-04-27 14:32:14 INFO config-changed nrpe_setup = nrpe.NRPE(hostname=hostname)
2018-04-27 14:32:14 INFO config-changed File "/var/lib/juju/agents/unit-hacluster-rados-1/charm/hooks/charmhelpers/contrib/charmsupport/nrpe.py", line 244, in __init__
2018-04-27 14:32:14 INFO config-changed self.nagios_context = self.config['nagios_context']
2018-04-27 14:32:14 INFO config-changed TypeError: 'NoneType' object has no attribute '__getitem__'

[Edward Hope-Morley (hopem)]

Patch submitted to https://github.com/juju/charm-helpers/pull/166

[Trent Lloyd (lathiat)]

This fix didn't appear to work (at least in some cases), new fix:
https://github.com/juju/charm-helpers/pull/484