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/ceilometer b/lib/ceilometer
index 8135309..dba92ba 100644
--- a/lib/ceilometer
+++ b/lib/ceilometer
@@ -315,6 +315,9 @@
         git_clone_by_name "ceilometermiddleware"
         setup_dev_lib "ceilometermiddleware"
     else
+        # BUG: this should be a pip_install_gr except it was never
+        # included in global-requirements. Needs to be fixed by
+        # https://bugs.launchpad.net/ceilometer/+bug/1441655
         pip_install ceilometermiddleware
     fi
 }