Allow hub to be client of other hub

[rkhamis]

Issue migrated from https://api.github.com/repos/zero-os/0-hub/issues/17, opened by @zaibon

The idea is to allow a hub to configure some fallback hub in the case what is requested doesn't exist locally.

Example use case:

• One person host a private hub for his own use, but still want to be able to grab all the public images from it.
• having hub that proxy the public hub to have it locally to your networks

Need to think about security, how the hub can get access to the data of the fallback hub

[rkhamis]

*commented by @maxux
The idea is:

• A hub can be configured as slave of another hub
• When a listing request is sent to the hub, it merge it's local contents with the master server(s)
• Only requesting/listing flist is propagated to master(s) (eg: deleting or renaming is only local)
• An efficient way would be better (not requesting remote data on each request)*