Pin oslo.messaging to stable/icehouse branch

When using the master branch with stable/icehouse, there is
a mismatch in requirements resulting in the following:
"'oslo.middleware' is not a global requirement but it should be,
  something went wrong"

This error does not occur if oslo.messaging is pinned to its
stable/icehouse branch.

Change-Id: I3b0b628756e48bdf0571fccaeff979086c08cff5