Image modifier hooks can't work with simplestreams snap

image-modifier subordinates (used for example by Launchpad's build farm infrastructure) fundamentally can't work with glance-simplestreams-sync's current approach of using the simplestreams snap. g-s-s currently doesn't pass --modify-hook to sstream-mirror-glance, but even if it did, it would rely on sstream-mirror-glance being able to run the modifier hook as a subprocess which it can't do since simplestreams isn't a classic snap.

Approaches I can think of:

 * go back to using .deb packaging for simplestreams, except presumably with a centrally-maintained PPA
 * consume simplestreams as a pip dependency using a virtualenv somewhere
 * vendor simplestreams