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/databases/postgresql b/lib/databases/postgresql
index a6bcf8c..e087a1e 100644
--- a/lib/databases/postgresql
+++ b/lib/databases/postgresql
@@ -104,7 +104,7 @@
 
 function install_database_python_postgresql {
     # Install Python client module
-    pip_install psycopg2
+    pip_install_gr psycopg2
     ADDITIONAL_VENV_PACKAGES+=",psycopg2"
 }