Enhance Nic.master for ovs virtual switches

Add a test to check whether an interface is part of a ovs virtual switch.
This is to stop the network barclamp from reconfiguring such interfaces,
similar to what is already does for linux-bridge and bond slaves.
Merge pull request #153 from rhafer/pull-req-release-pebbles-master-fe6235d7a36ecb5fb03d7dc3628236c4743f2296

Enhance Nic.master for ovs virtual switches [1/2]