Fix dhcp_agent configuration for vmware DVS

interface_driver parameter is missing when using vmware DVS on the
dhcp_agent configuration. As a result we get the following error:
Error loading interface driver 'None'

Change-Id: I302a1392615792bd65cab9c0933cb7f7e5e1880d
Closes-Bug: #1736410

ansible/roles/neutron/templates/dhcp_agent.ini.j2

@@ -7,6 +7,7 @@ dnsmasq_dns_servers = 8.8.8.8,8.8.4.4
 
 {% if neutron_plugin_agent == 'vmware_dvs' %}
 ovs_integration_bridge = {{ neutron_bridge_name }}
+interface_driver = openvswitch
 enable_metadata_network = True
 dhcp_driver = vmware_nsx.plugins.dvs.dhcp.Dnsmasq
 use_namespaces = True