Fix constraint usage for unmaintained/victoria|wallaby

vistoria and wallaby branches are renamed to unmaintained/
namespace. Fixing run-tempest role to adopt those new name
for those branches so that it can set the correct constraints
while installing the tempest.

Change-Id: I595c3530461173fe3c9ba258aeb1ae90314874bf
diff --git a/roles/run-tempest/README.rst b/roles/run-tempest/README.rst
index 04db849..c682641 100644
--- a/roles/run-tempest/README.rst
+++ b/roles/run-tempest/README.rst
@@ -81,7 +81,7 @@
 .. zuul:rolevar:: stable_constraints_file
    :default: ''
 
-   Upper constraints file to be used for stable branch till stable/victoria.
+   Upper constraints file to be used for stable branch till Wallaby
 
 .. zuul:rolevar:: tempest_tox_environment
    :default: ''