Allow rpc drivers to always be imported

The qpid and zmq drivers current can't be import unless their
respective libraries are installed and neither are currently listed
in test-requires.

This prevents us from being able to use nosetests --with-doctests
because the import failure is seen as a test failure.

Fix this issue by making the import always succeed and deferring
failure until the module's APIs are actually used.

This will also help Nova's sample config file generation since it
will allow the generate-sample tool discover config options for these
drivers even when the messaging libraries aren't installed.

Change-Id: I30087fd33eef41d50d1a374cb5694dc15c43d1da
Merge "Allow rpc drivers to always be imported"