doc: Add USE_PYTHON3 to configuration guide
doc: Add USE_PYTHON3 to configuration guide

Change-Id: I8b6743c1dbdc00001691b2727bd0c344fb6ccd51

noironetworks/devstack,dtroyer/devstack,dtroyer/devstack,openstack-dev/devstack,noironetworks/devstack,group-policy/devstack,group-policy/devstack,openstack-dev/devstack,nawawi/openstack,nawawi/openstack