Extract the .dockercfg code 

from registry/auth.go to a new config.config.go

I've also removed some legacy conversion from a format pre 0.5.3.

The second commit is the reason I've done this - we need a way for boot2docker to be able to default the DOCKER_HOST setting - and on discussion with @shykes, modifying our config file is a better option than modifying the profile and setting an ENV.