Call babysit() synchronously on worker startup.

When each worker becomes available Pulp will be notified
via Celery's signal system. Our new signal handler will
call Pulp's initialization() code, and then will call
babysit() synchronously.
Add a test for when active_queues() is None.

babysit() used to blow up if active_queues() was None. Now it checks to
see if it is None before using it.
Merge branch 'rbarlow-babysit_during_init'