fix(registry): store apps as appname

Storing the image in the registry as user/appname introduces a couple
of confusing ideas:

- images are implied to be unique across accounts
- images are owned by one user and one only

Point 1 is incorrect. App names are completely unique across the
entire cluster[1]. Point 2 is incorrect as well because apps can
be shared with the `deis perms` command. Changing the name of the
image in the registry from `user/appname` to `appname` makes the
image names unique across the entire cluster as well as making the
images more easily sharable between users.

fixes #729

[1]: https://github.com/deis/deis/blob/d1e14a6f76789a86f1959ba7bf6057b3abe3b960/controller/api/models.py#L109