base-test: use gather_subset: network for ansible 2.3
base-test: use gather_subset: network for ansible 2.3

It seems gather_subset: !all,network doesn't work on 2.3, but does on
2.4. Since our executors are only 2.3, switch to this syntax. In
locally testing this works as expected.

Change-Id: Ie3e04e8f1e8e8ce09632274d20609c6c9e665a04
Signed-off-by: Paul Belanger <e7139c8e6c4ea75cc1c287ce081b2dc0f3ab39ec@redhat.com>

openstack-infra/project-config,openstack-infra/project-config