Fix setup of Neutron core plugin in VPNaaS UT

One of the VPNaaS unit test setup routines creates
and extension manager but passes it a class but it
should be passing an instance of that class.

Change-Id: I589cdda0674fdf8fa20d92c2609e1ba6966125d8
Merge "Fix setup of Neutron core plugin in VPNaaS UT"