python-tempestconf will prioritize the external network named public
when auto-discovering an external network for tempest.
The public network is the network we end up using in most of the
cases. This will help avoid potential conflicts with other available
external networks that are not meant to be used by tempest. If no
external network with name public is found, the network discovered
last will be used.
Change-Id: Ib10cae1fa3610c4ea0a54b7f4ebbdb146bf4fce2
e4d8dc124c