Sync up the default value for the network_for_ssh option

The code and tempest.conf.sample were using different defaults for the
network_for_ssh option. This patch makes tempest.conf.sample align with
the default in the code.

Fixes bug 1207082

Change-Id: Iac990b35f904259ef81a5b865d8b8be6e5c30822
Merge "Sync up the default value for the network_for_ssh option"