Fixed pass bug in dell driver
If a vlan is not in the DB when the driver tries to remove it, an error is
logged and an exception is thrown.
Also, a test was added to test this functionality.
free_network_id no longer throws an exception
Also, added some comments to the unit test for this function.
Removed unneeded import
Merge pull request #230 from ryabouza/dell-pass-bugfix

Fixed pass bug in dell driver