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/hypervisor-xenserver b/lib/nova_plugins/hypervisor-xenserver
index 4d0ec89..efce383 100644
--- a/lib/nova_plugins/hypervisor-xenserver
+++ b/lib/nova_plugins/hypervisor-xenserver
@@ -94,7 +94,7 @@
 
 # install_nova_hypervisor() - Install external components
 function install_nova_hypervisor {
-    pip_install xenapi
+    pip_install_gr xenapi
 }
 
 # start_nova_hypervisor - Start any required external services