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
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 825965f..29f99f2 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -90,7 +90,7 @@
 fixed_network_name = private
 
 # Network id used for SSH (public, private, etc)
-network_for_ssh = private
+network_for_ssh = public
 
 # IP version of the address used for SSH
 ip_version_for_ssh = 4