Fixes Bug 90343: tuned rabbitmq crashstorage to work better with submitter_app

when socorro is configured with RabbitMQ, there needs to be a way to submit crashes to the priority queue.  Eventually, we want a Middleware service for it, but until then, the submitter_app would work fine.  We want the submitter to be able to do something like this:

> submitter_app.py 
   --source.crashstorage_class=DBSamplingCrashSource 
   --source.sql="select uuid from reports where signature='foo'"
   --destination.crashstorage_class=socorro...rabbitmq...RabbitMQCrashStorage
   --destination.routing_key=socorro.priority
   --destination.filter_on_legacy_processing=False

The change to the code is confined to the RabbitMQ storage class.  

the first change was to devise a way for the crash storage class to use a queue other than 'socorro.normal'.  This was done by making it a configman parameter.

the second change was to get the save_raw_crash to ignore the 'legacy_processing' flag within a crash. This was also done with a configman parameter that tells the class to use or ignore that flag.

other changes are clean ups: caching a single instance of the queue properties object instead of creating a new one for each submission; and the addition of more tests