Switch py34 tests to py35
Switch py34 tests to py35

Py35 is now commonly available on things like -infra test nodes, lets
use that instead.

Change-Id: I554ec84d709a9937df5ec3c2fa7eab8f158cf64e

openstack/diskimage-builder,switch-ch/diskimage-builder,switch-ch/diskimage-builder,openstack/diskimage-builder