Add deprecated_reason for nova_cert

Since Id7a1fc943cbe6d860a50d3cc776717b55351004b , the service has
been deprecated on Nova side. So this patch adds the reason to Tempest
to know when we can remove this option.

Change-Id: Icd9c1a27a998df15c1953427a91ae46f8fc5647e
diff --git a/tempest/config.py b/tempest/config.py
index bc9215c..95a94d9 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -397,7 +397,10 @@
     cfg.BoolOpt('nova_cert',
                 default=False,
                 help='Does the test environment have the nova cert running?',
-                deprecated_for_removal=True),
+                deprecated_for_removal=True,
+                deprecated_reason="On Nova side, the nova-cert service is "
+                                  "deprecated and the service will be removed "
+                                  "as early as Ocata."),
     cfg.BoolOpt('personality',
                 default=False,
                 help='Does the test environment support server personality'),