Tempest API tests: Add ipv6 attribute tests

Implements blueprint ipv6-subnet-attributes

Change-Id: I7f8358f4fc507cc2ea73ebbf8a7ff25f0e8306fe
diff --git a/tempest/config.py b/tempest/config.py
index 6d9fda6..7a0f4c4 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -432,6 +432,12 @@
                 default=['all'],
                 help='A list of enabled network extensions with a special '
                      'entry all which indicates every extension is enabled'),
+    cfg.BoolOpt('ipv6_subnet_attributes',
+                default=False,
+                help="Allow the execution of IPv6 subnet tests that use "
+                     "the extended IPv6 attributes ipv6_ra_mode "
+                     "and ipv6_address_mode"
+                )
 ]
 
 queuing_group = cfg.OptGroup(name='queuing',