Fixes bug 1049385
Due to some issues using OVS internal interfaces across namespaces
with OpenFlow controllers (bug 1048681), a patch introduced the
OVSVethInterfaceDriver in addition to the base OVSInterfaceDriver.
However, OVSVethInterfaceDriver is just a variation of OVSInterfaceDriver
and the difference is how to create an interface (OVS internal vs veth).
This patch merge OVSVethInterfaceDriver into OVSInterfaceDriver
by introducing a new flag 'ovs_use_veth' (which defaults to False).
Change-Id: Ie8b01e6776bf703f72a9e2a471b24e126f6e2322