Update oslo.messaging to 1.4.0.0a3

This oslo.messaging version fixes a problem with encoding
issue in source file(review 102909), causing some Neutron
gate test failures (e.g. 102351). Using a3, since it is the
latest.

Change-Id: I673b5d505c8b7d43d59f7516e345f92221483a51
Merge "Update oslo.messaging to 1.4.0.0a3"