Install python-novaclient 2.15.0 and newer on PY3

All versions before 2.15.0 are not compatible with python 3.3. This
change will restrict to that version or newer for python 3 only. PBR
will look at requirements-py3.txt, if present, and install a newer version
of python-novaclient.

Change-Id: I00298dcd2e479f0683bf7884e68e88912a5cb1b7
Implements: blueprint relaxed-novaclient-version
Merge "Install python-novaclient 2.15.0 and newer on PY3"