Add py34 to tox
Add py34 to tox

We gate on py34, so we shoudl add it to our tox config.

Change-Id: I0d21cdd47841af8d05df7b36b2302f13bda3367b

CSC-IT-Center-for-Science/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder,zioproto/diskimage-builder,rdo-management/diskimage-builder,rdo-management/diskimage-builder,switch-ch/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder,takahashinobuyuki/diskimage-builder