install: Run container fetch in host context

[cgwalters]

Moved from #919 (comment)

We already hit in the past a problem where a user had configured their storage.conf on the host to use a different location (i.e. not /var/lib/containers) and this breaks things right now because our bind mount setup assumes /var/lib/containers.

The logistical annoyance here is that we already were doing this correctly before by forking off skopeo in the host mount namespace. Except we hit constant problems where people had podman but not skopeo. And the image proxy API is only in skopeo, not podman.

So fixes:

• Add the image proxy to podman too
• Add Go code here to talk directly to c/image and c/storage (ref Unified storage #20 )