Fix getaddrinfo if dnspython is installed

If dnspython is present in the environment then eventlet
monkeypatches socket.getaddrinfo() with an implementation
which doesn't work for IPv6.

This patch has also been applied to other Openstack
projects such as nova (f2e49ad3) and keystone (3afd9791)
in order to work around the same problem and is
intended to be used in the same way.

Co-authored-by: Hui Xiang <hui.xiang@canonical.com>
Closes-Bug: 1331885
Change-Id: I795e004eac3f032217ff9cb0047f1976306fbb71
Merge "Fix getaddrinfo if dnspython is installed"
Fix and add a test case for IPv6

Fixing and adding a test case in relation to the eventlet ipv6 fix
in patch set 23 (https://review.openstack.org/#/c/101079/).

  * pre-import eventlet for test case test_evnetlet_no_dnspython,
    which wasn't actually importing eventlet like it described.
  * add case test_no_evnetlet_no_dnspython

Change-Id: I55c40ed735781862f8352afb917058bf475d16fe