remove skip for test_connectivity_between_vms_on_different_networks
remove skip for test_connectivity_between_vms_on_different_networks

test test_connectivity_between_vms_on_different_networks was skipped due
to bug https://bugs.launchpad.net/tempest/+bug/1610994 which seems no
longer valid. I run the test on a valid environment and its passing.

Change-Id: I8c607758cb98978c0cda92b2b14467a1a972d33b

openstack/tempest,cisco-openstack/tempest,masayukig/tempest,masayukig/tempest,cisco-openstack/tempest,openstack/tempest