Code review comment for ~lutostag/layer-snap:use_juju_proxy

Otherwise all looks good, except that I'd like to avoid mutating the hookenv.config() dictionary and instead keep using the charms.reactive data_changed() helper. charmhelpers.hookenv needs to be reworked into charms.hookenv at some point, and the mutating config has been deprecated in favor of charmhelpers.unitdata (which data_changed uses under the hook).