Enable the ability to push configs for existing deals

[andrewxhill]

Scenario

• User creates a deal offline (see https://github.com/textileio/powergate/issues/399)
• They would then like to use the Powergate to monitor long term status of that deal
• They may want to use the Powergate to subselect specific files from that deal to store in hot (see https://github.com/textileio/powergate/issues/400)

I think that would be possible.
It could made easier if Filecoin had some way to discover which miner has some data... since now the only discovery that exist is just see if the Lotus node made the deals. But that might be solved if apart from importing the Cid in question, you can also import the existing deals, and then move on from there as it was originally pushed by PG.

[asutula]

Looks like we now have support in FFS for importing: https://github.com/textileio/powergate/blob/master/ffs/api/api_import.go#L19

@jsign and @andrew is there any reason we shouldn't move forward with exposing this feature in the API?

[jsign]

I created it some time ago while working on single-retrievals and Dumbodrop project. Never really tested in a live network and don't recall any missing part (I think there aren't)... but may be a possibility.