Add new function get_from_global_requirements

Instead of specifying the version of a library in devstack, use the version from
global-requirements

Add new function get_from_global_requirements and use it
where it makes sense.

Change-Id: I6b2f062761ac05ef72fc6cc9993bc204faf06fa5
diff --git a/lib/tempest b/lib/tempest
index 5ca217e..bb08a05 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -96,7 +96,8 @@
 # configure_tempest() - Set config files, create data dirs, etc
 function configure_tempest {
     # install testr since its used to process tempest logs
-    pip_install `grep -h testrepository $REQUIREMENTS_DIR/global-requirements.txt | cut -d\# -f1`
+    pip_install $(get_from_global_requirements testrepository)
+
     local image_lines
     local images
     local num_images