Update to python-neutronclient 2.3.6

Nova needs python-neutronclient >= 2.3.6
for the new MacAddressInUseClient exception.

Change-Id: Id1b82f27ae09503ba3f4f92e5fa77f28b76d1580
Related-Bug: #1347778
Merge "Update to python-neutronclient 2.3.6"