Use stevedore aliases for interface_driver configuration

interface_driver configuration was updated to use stevedore aliases.
This patch is to change devstack scripts to now use the aliases
instead of the previous class imports.

Change-Id: Ida10765f89c07205837e46f8cb445391f0e15373
Closes-Bug: #1504536

ansible/roles/neutron/templates/neutron.conf.j2

@@ -23,9 +23,9 @@ metadata_workers = {{ openstack_service_workers }}
 metadata_proxy_socket = /var/lib/neutron/kolla/metadata_proxy
 
 {% if neutron_plugin_agent == "openvswitch" %}
-interface_driver = neutron.agent.linux.interface.OVSInterfaceDriver
+interface_driver = openvswitch
 {% elif neutron_plugin_agent == "linuxbridge" %}
-interface_driver = neutron.agent.linux.interface.BridgeInterfaceDriver
+interface_driver = linuxbridge
 {% endif %}
 
 {% if enable_nova_fake | bool %}