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/ldap b/lib/ldap
index d69d3f8..d2dbc3b 100644
--- a/lib/ldap
+++ b/lib/ldap
@@ -142,7 +142,7 @@
         sudo ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/inetorgperson.ldif
     fi
 
-    pip_install ldappool
+    pip_install_gr ldappool
 
     rm -rf $tmp_ldap_dir
 }