Warn OPENSTACK_QUANTUM_NETWORK setting as deprecated

Change-Id: If2f762fe665b9a88153a77a658f52bcd56185c53
Closes-Bug: #1377498
(cherry picked from commit 530e5fee789ce5ed19d90a6b4901f01e8efde5ff)
Merge "Warn OPENSTACK_QUANTUM_NETWORK setting as deprecated" into proposed/juno