Only configure DHCP for real interfaces

Updates the generate-interfaces-file.sh script in
the dhcp-all-interfaces element so that we only
add interfaces that have real MAC addresses.

The generate-interfaces-file.sh script is run early enough
in the boot process (before OVS is initialized) that this
isn't usually a problem unless you execute it manually by hand
after booting. Then you'll end up with network/DHCP
configs for all of your OVS bridges, etc. This
patch avoids configuring all of the virtual interfaces
which have generated MAC addresses.

Change-Id: I7a705084aa5b11305ac0ec5ca37fd2e87a2ae8b7
Closes-bug: 1239479
Merge "Only configure DHCP for real interfaces"