Mock the udevadm in the TunnelTestWithMTU test

When running the neutron.tests.unit.openvswitch.test_ovs_tunnel.
TunnelTestWithMTU unit test failure occurs because the udevadm is
timing out waiting on a mock-ed device.

Co-Authored-By: Salvatore Orlando <salv.orlando@gmail.com>

Change-Id: I0e16295be66513c2257831bf62fd6a0c19f3b8a4
Closes-Bug: #1253506
Merge "Mock the udevadm in the TunnelTestWithMTU test"