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

BREAKING CHANGES: because this changes the way that applications
are stored in the registry, this is a backwards-incompatible change
with applications prior to this commit. To migrate over to this change,
you'll have to destroy and re-create your apps with the same name
as it was before. The docker layers from previous builds will still
exist in the registry, but the repository names will be changed
to reflect this change.
fix(controller): remove username length validation

This fix that was implemented in #631 was a bit hackish, and was
only used because we stored the images as username/appname in the
registry. The registry has a restriction where the username has
to be greater than or equal to 4 in length. Now that we are not
storing the images with the associated owner's username, we no
longer need this restriction.

reverts #631