Due to python-libjuju's requirement for coercing every config
value into strings, some configuration type/values are not
possible to specify the default for.
Allow the config_change helper to optionally use reset back to
charm default instead of specifying default config values in a
dictionary.
When configuring the address to use to talk to Keystone,
the format_addr helper should be used to ensure that an
IPv4 or IPv6 address can be correctly handled.