ml2 is new icehouse core plugin default

Update core plugin default to ml2.
Replace plugin_conf_map with core_plugin_map since basing it upon
interface driver does not work for ml2.
Cleanup interface_driver settings for ml2, it should default to
openvswitch interface.
Update tests.

Implements: blueprint neutron-icehouse-default-changes
Change-Id: I934d20258beac779da5e53147571c0c2b1149563