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/horizon b/lib/horizon
index 122d516..a8e83f9 100644
--- a/lib/horizon
+++ b/lib/horizon
@@ -182,8 +182,7 @@
 # NOTE: It can be moved to common functions, but it is only used by compilation
 # of django_openstack_auth catalogs at the moment.
 function _prepare_message_catalog_compilation {
-    local babel_package=$(grep ^Babel $REQUIREMENTS_DIR/global-requirements.txt)
-    pip_install "$babel_package"
+    pip_install $(get_from_global_requirements Babel)
 }