Perform sort when creating hostnames_ip file

Perform a sort when creating the openstack_hostnames_ips.yml file so
that this file is easier to read and is generated consistently between
runs of Ansible playbooks and commands.

Change-Id: I192fd8162c1da90c79f62524d5b92d96f11e9125

playbooks/inventory/dynamic_inventory.py

@@ -966,7 +966,8 @@ def main():
         f.write(
             json.dumps(
                 hostnames_ips,
-                indent=4
+                indent=4,
+                sort_keys=True
             )
         )