Kill ability to specify exchange in transport URL

My original thinking was that if you're using the exchange name to
separate two instances of the applications (so their queues don't
collide) then the exchange name is pretty key to transport
configuration. In fact, it's really a virtual host that you'd use for
this (at least in the case of rabbit and qpid).

Also, Nova's cells code has already moved ahead with the assumption that
the path specifies a virtual host, so it'd only make sense to deviate
from that if there was a really good reason to.

Change-Id: Ic8b5dc3538b6b17afec524047acc2efa76366377
Merge "Kill ability to specify exchange in transport URL"