Network API: default to ipv4, add ipv6 tests

Change-Id: I34da0ede4d7b92b2e752c2da82172c495e1946b1
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 990cb37..913f41c 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -525,13 +525,20 @@
 # value)
 #region=
 
-# The cidr block to allocate tenant networks from (string
+# The cidr block to allocate tenant ipv4 subnets from (string
 # value)
 #tenant_network_cidr=10.100.0.0/16
 
-# The mask bits for tenant networks (integer value)
+# The mask bits for tenant ipv4 subnets (integer value)
 #tenant_network_mask_bits=28
 
+# The cidr block to allocate tenant ipv6 subnets from (string
+# value)
+#tenant_network_v6_cidr=2003::/64
+
+# The mask bits for tenant ipv6 subnets (integer value)
+#tenant_network_v6_mask_bits=96
+
 # Whether tenant network connectivity should be evaluated
 # directly (boolean value)
 #tenant_networks_reachable=false