Update tempest version required

I093be875910ef18deebe3e56b83c6992d1a307e5 introduced the
use of newer code from tempest that isn't available until
the 31.1.0 release.

Change-Id: Ibe2946fafd4d2b58d49b6c4040887346df1f9899
Related-Bug: #1968069
Signed-off-by: Goutham Pacha Ravi <gouthampravi@gmail.com>
diff --git a/requirements.txt b/requirements.txt
index 6922d70..95ad247 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,4 +6,4 @@
 
 ddt>=1.0.1 # MIT
 oslo.log>=3.36.0 # Apache-2.0
-tempest>=27.0.0 # Apache-2.0
+tempest>=31.1.0 # Apache-2.0