Stop running L3 functional tests with both OVSDB interfaces

Stop running L3 functional tests with both OVSDB interfaces

Running the L3 functional tests with both OVSDB interfaces doubles
the run time and may discourage developers from running them
frequently during development. Since the OVSDB interfaces
are tested explicitly, I don't think the trade off is worth it
here. The L3 functional tests use OVS in a *really* trivial way
and won't catch any issues that the explicit tests won't.

Added an OVSInterfaceDriverTestCase plug functional test that runs with
both OVS interfaces to make it harder to introduce regressions.

Related-Bug: #1442272
Change-Id: I387db347fe34f8497069ddf768624bccb9d1de8b