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/nova_plugins/functions-libvirt b/lib/nova_plugins/functions-libvirt
index d4a0768..04da5e2 100755
--- a/lib/nova_plugins/functions-libvirt
+++ b/lib/nova_plugins/functions-libvirt
@@ -31,12 +31,12 @@
             install_package python-guestfs
         fi
         install_package libvirt-bin libvirt-dev
-        pip_install libvirt-python
-        #pip_install <there-si-no-guestfs-in-pypi>
+        pip_install_gr libvirt-python
+        #pip_install_gr <there-si-no-guestfs-in-pypi>
     elif is_fedora || is_suse; then
         install_package kvm
         install_package libvirt libvirt-devel
-        pip_install libvirt-python
+        pip_install_gr libvirt-python
         install_package python-libguestfs
     fi