celerypylons: wrap celery import so we always get the right environment variables set and check configuration

Get rid of magic ... or at least document and encapsulate it.

Before, celerypylons would set the environment variables that made it possible
to import Celery after Pylons had been configured.

Now, the module will import Celery ... and verify that Pylons has been
configured.

(A next stop could be to move things around so this got tied closely to
initializing Pylons. Or something.)