Only initialize remote artifact cache connections if needed

This fixes a regression from the canonical-pull-urls branch that was
just merged. The OSTreeCache.__init__() function was connecting to
the cache, which is bad because execution would randomly freeze for
several seconds when the connection was slow.

We now only initialize remote connections where needed; this was
already introduced in 5c2ef6d0 but
had regressed. I renamed the keyword arg from `fetch_remote_refs`
to `use_remote_cache` because it needs to be set for any interaction
with the remote cache, doesn't matter if they are fetches or pushes.