Make add ip_floating_ip idempotent

Closes issue #1858

lib/ansible/modules/cloud/openstack/os_floating_ip.py

@@ -162,6 +162,10 @@ def main():
                 msg="server {0} not found".format(server_name_or_id))
 
         if state == 'present':
+            fip_address = cloud.get_server_public_ip(server)
+            f_ip = _get_floating_ip(cloud, fip_address)
+            if f_ip:
+                module.exit_json(changed=False, floating_ip=f_ip)
             server = cloud.add_ips_to_server(
                 server=server, ips=floating_ip_address, ip_pool=network,
                 reuse=reuse, fixed_address=fixed_address, wait=wait,