fix(controller): store limit fields on config

Having a separate "Limit" domain object was getting in our way, when
really it represents just an extension of a Config. This moves the
two JSON fields "cpu" and "memory" to Config and does away with Limit.

Also added a regression unit test to ensure that `deis config:set` doesn't 
wipe out existing limits.

Reverted code changes in #1616 as that wasn't the right fix, but kept the
tests intact.

Fixes #1613.