Add deprecation notices to the old nova inventory

plugins/inventory/nova.ini

 # Ansible OpenStack external inventory script
+# DEPRECATED: please use openstack.py inventory which is configured for
+# auth using the os-client-config library and either clouds.yaml or standard
+# openstack environment variables
 
 [openstack]
 

plugins/inventory/nova.py

@@ -17,6 +17,10 @@
 # You should have received a copy of the GNU General Public License
 # along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
 
+# WARNING: This file is deprecated. New work should focus on the openstack.py
+# inventory module, which properly handles multiple clouds as well as keystone
+# v3 and keystone auth plugins
+
 import sys
 import re
 import os
@@ -28,6 +32,9 @@ try:
 except ImportError:
     import simplejson as json
 
+
+sys.stderr.write("WARNING: this inventory module is deprecated. please migrate usage to openstack.py\n")
+
 ###################################################
 # executed with no parameters, return the list of
 # all groups and hosts