add docker__daemon_json_dict setting

[waynr]

See #93

[nickjj]

I like the idea in general but the implementation would need a number of changes.

There's both a docker__default_daemon_json and docker__daemon_json variable that get merged together so we'd want to use combine() for that. Then there's updating the tests too.

I'm also not a fan of keeping around 2 documented ways to configure the same thing or the name of the variables themselves. Perhaps it could now be docker__daemon_options. If it were named that originally we could have gotten by with having both 1 set of variables and backwards compatibility at the jinja template level (if the var is a string do the old thing else do the new thing) but that ship has sailed.

I think the only real option is to bite the bullet and make a backwards incompatible change.