Update global-requirement for tempest-lib

The latest tempest-lib contains compute service clients.
This patch bumps the version for the latest to use these clients.

Change-Id: I03988620566342c6906f47e9044053f0fa89fb1d
Depends-on: Ifa69e6c0ecc12d47bb4c53b9b436c2463f1932e2
diff --git a/requirements.txt b/requirements.txt
index df04ca8..1f225c3 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -21,6 +21,6 @@
 iso8601>=0.1.9
 fixtures>=1.3.1
 testscenarios>=0.4
-tempest-lib>=0.11.0
+tempest-lib>=0.12.0
 PyYAML>=3.1.0
 stevedore>=1.5.0 # Apache-2.0