introduce pip_install_gr

This creates a new pip_install_gr that installs from global
requirements allowed versions. Now that stable branches are getting
capped all of devstack needs to be fixed to do things like this.

Change-Id: I8fd0ef2bfc544ca2576fab09d3018f760b8848fe
diff --git a/lib/horizon b/lib/horizon
index 63a9d0f..f953f5c 100644
--- a/lib/horizon
+++ b/lib/horizon
@@ -183,7 +183,7 @@
 # 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 {
-    pip_install $(get_from_global_requirements Babel)
+    pip_install_gr Babel
 }