Self assign controller MAC when cannot retrieve one from host

- When Switch Manager is not able to retrieve the list of the host
  network interfaces, self assign a custom MAC address.
  This is a quick fix for Hydrogen release. User can still
  overwrite the custom MAC with the one of his choice via
  switchmanager REST API

Change-Id: I7a76107fc970e8dead649ac9e8703533921c8d30
Signed-off-by: Alessandro Boch <aboch@cisco.com>