Code review comment for lp://staging/~freyes/charms/trusty/glance-simplestreams-sync/lp1522929

@mikemc Charms should not install PPAs (some people are doing offline installs and want to control apt/image sources without interference from the charm). If this is absolutely necessary then it should at least be an option e.g. the ceph charm allows an alternate source to be installed with the 'source' option.