Add check on eventlet bug #105 (ipv6 support)

If our environment contain package dnspython and have ipv6 enabled in
the system, we have a problem. We have a couple of tests that
fail in such environment.

Add utils.is_eventlet_bug105 function to test environment on this
issue. Also add utils.is_ipv6_configured(moved from tests
.test_wsgi.TestWSGIServer._ipv6_configured).

Add additional constraints to ipv6 tests.

Quick workaround for problem - set environment variable
EVENTLET_NO_GREENDNS.

Change-Id: I9e91ffcca225a556842e97db07dbb0edaddf8393
Merge "Add check on eventlet bug #105 (ipv6 support)"