zmq: switch back to not using message envelopes

In oslo-incubator, we default to not serializing messages using a
message envelope:

  def _cast(.., envelope=False, ...)

except using the v2 notifyer which explicitly passes envelope=True.

However, in oslo.messaging, we accidentally switched to using a
message envelope:

     def _send(self, ..., envelope=True):

This was not intentional and may be disruptive to users.

This issue was identified because the envelope=True path just doesn't
work! We don't have any zmq tests in oslo.messaging, but even in
oslo-incubator the code path isn't tested and has been broken since
I73aad7697cf83ad4aabb3c2058b7cc4f53f783c2

Partial-Bug: #1301132
Change-Id: I6aa2e2ac3cebf443719d64ebcd9eaadc0a9328b8
Merge "zmq: switch back to not using message envelopes"