Periodically poll registries and pull images according to sync's config
Added sync on demand, syncing when clients asks for an image which
zot doesn't have.
Signed-off-by: Petu Eusebiu <peusebiu@cisco.com>
added support to point multiple storage locations in zot by running multiple instance of zot in background.
see examples/config-multiple.json for more info about config.
Closes#181