validate flat networks physical name

Modified flat driver to validate the physical network
name - we make sure that empty names are deducted.

unit test changes:
- added two tests to make sure that we detect empty
  physnet names
- fix bug in existing unit-tests
-- flat_networks should be a list of string and not string
- use conf to setup flat_network

Change-Id: Ib7cfbd3c97d2e95c82044a2cba683c917ea54a84
Partial-bug: #1325664
Merge "validate flat networks physical name"