Add gabbi requirement for API testing in Ceilometer

Ceilometer is using gabbi to drive functional testing of its API.

The minimum version requirement is for required fixture support and
pattern matching response headers.

Change-Id: I0f828c1ec184bb9a6760bb18eaafb9c91550d4db

global-requirements.txt

@@ -28,6 +28,7 @@ extras
 falcon>=0.1.6,<0.2.0
 Flask>=0.10,<1.0
 futures>=2.1.6
+gabbi>=0.3.0
 gear
 glance_store>=0.1.10                   # Apache-2.0
 greenlet>=0.3.2