Allow IPv6 tests to be disabled

By default they are enabled. This is useful if
certain plugins do not fully support IPv6, or if
the 3rd party CI is set up for IPv4 only.

Closes-bug: 1282387

Change-Id: I6d402381474d70c5f7a4fe378c0459c9b3a92085
Merge "Allow IPv6 tests to be disabled"
Merge branch 'master' of https://github.com/openstack/tempest