fix(controller): fix NoneType config values

if publish_release is called without configuration data (i.e.
config = None), the loops in _generate_env fail to work properly
because a NoneType in Python is not an iterable type.

TESTING: rebuild the controller with this change and push an app:

    $ deis create test
    $ git push deis master

then, test that a 'v2' tag was successfully created in the registry:

    $ curl local.deisapp.com:5000/v1/repositories/test/tags
    {"latest": "ed14150269724148851d54d5f3ed104ced14150269724148851d54d5f3ed104c",
     "v2": "ed14150269724148851d54d5f3ed104ced14150269724148851d54d5f3ed104c"}

fixes #950