Remove dragonflow ML2 driver

Dragonflow is no longer maintained as an OpenStack project [1]
and has therefore been removed from OpenStack-Ansible as a
supported ML2 driver for neutron.

[1] https://review.openstack.org/613856

Change-Id: Ia7042e5dd697611ef4d9148b6f345d5da887b2c5

tox.ini

@@ -120,15 +120,6 @@ commands =
     bash -c "{toxinidir}/tests/common/test-ansible-functional.sh"
 
 
-[testenv:dragonflow]
-setenv =
-    {[testenv]setenv}
-    ANSIBLE_INVENTORY={toxinidir}/tests/dragonflow_inventory
-    ANSIBLE_OVERRIDES={toxinidir}/tests/neutron-overrides-dragonflow.yml
-commands =
-    bash -c "{toxinidir}/tests/common/test-ansible-functional.sh"
-
-
 [testenv:opendaylight]
 setenv =
     {[testenv]setenv}