openstack: only consider IPv4 subnets
openstack: only consider IPv4 subnets

Signed-off-by: Loic Dachary <5971e5274f1f0ba5329cf4a93393d8fbe5224c44@redhat.com>

ceph/teuthology,dreamhost/teuthology,caibo2014/teuthology,ktdreyer/teuthology,dmick/teuthology,SUSE/teuthology,SUSE/teuthology,dmick/teuthology,robbat2/teuthology,caibo2014/teuthology,robbat2/teuthology,dreamhost/teuthology,ceph/teuthology,SUSE/teuthology,ktdreyer/teuthology,dmick/teuthology