Remove asserts for optional imports

Horizon's __init__.py has some code that explicitly allows it to
be imported by setup.py before Django is set up, by making import
failures just throw a warning instead of an exception. But that code
is defeated by a bunch of asserts that were added later, that just
make the error even more confusing.

I'm removing the asserts and replacing them with __all__.

Closes-bug: #1210253
Change-Id: I0fde7810a033fd7216da9a5460f5e4e323b4d6e9
Merge "Remove asserts for optional imports"