Update python-neutronclient>=2.3.5,<3

Nova has a few bug fixes in progress that are using the OverQuotaClient
exception class defined in python-neutronclient-2.3.5 so bump the
minimum version.

Related-Bug: #1331353
Related-Bug: #1210598
Related-Bug: #1301117

Change-Id: I0a8de70d8362380adc330ecbb41275755362bd52