Configure 'stp' per l2_network_device
Configure 'stp' per l2_network_device

- use 'stp' configured for each l2_network_device
- use 'stp' option from driver if it wasn't set for the
  l2_network_device (defailt)

Change-Id: I3fc5b9d2267f94310374de962072cf80e40375bf
Closes-Bug:#1608518

stackforge/fuel-devops,stackforge/fuel-devops