Added comment how to get list of network extensions in tempest.conf

Change-Id: I1f0d819e0f9a73d0477a3665a9e6fc4394f69f72
Closes-bug: #1428942
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index b3aca42..467c5aa 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -792,7 +792,8 @@
 
 # A list of enabled network extensions with a special entry all which
 # indicates every extension is enabled. Empty list indicates all
-# extensions are disabled (list value)
+# extensions are disabled. To get the list of extensions run: 'neutron
+# ext-list' (list value)
 #api_extensions = all
 
 # Allow the execution of IPv6 subnet tests that use the extended IPv6
diff --git a/tempest/config.py b/tempest/config.py
index 603ccd2..7db66f5 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -512,9 +512,10 @@
                 help="Allow the execution of IPv6 tests"),
     cfg.ListOpt('api_extensions',
                 default=['all'],
-                help='A list of enabled network extensions with a special '
-                     'entry all which indicates every extension is enabled. '
-                     'Empty list indicates all extensions are disabled'),
+                help="A list of enabled network extensions with a special "
+                     "entry all which indicates every extension is enabled. "
+                     "Empty list indicates all extensions are disabled. "
+                     "To get the list of extensions run: 'neutron ext-list'"),
     cfg.BoolOpt('ipv6_subnet_attributes',
                 default=False,
                 help="Allow the execution of IPv6 subnet tests that use "