fix(controller): fix NoneType config values

if `Release.new` is called without new configuration data (i.e.
config = None), the if statements will be skipped because a
NoneType in Python means that the variable still has a value,
which is not the intended behavior. Explicitly checking if
config or build is a NoneType fixes this behavior.

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

controller/api/models.py

@@ -419,9 +419,9 @@ def new(self, user, config=None, build=None, summary=None):
 
         Releases start at v1 and auto-increment.
         """
-        if not config:
+        if config is None:
             config = self.config
-        if not build:
+        if build is None:
             build = self.build
         # prepare release tag
         new_version = self.version + 1