'Adds a firewall policy to the server\'s IP address
:param server_id: Id of the server
:rtype: ``str``
:param ip_id: ID of the IP address
:rtype: ``str``
:param firewall_id: ID of the firewall policy
:rtype: ``str``
:return: ``Node`` instance
:rtype: ``Node``'

def ex_add_server_firewall_policy(self, server_id, ip_id, firewall_id):

    body = {'id': firewall_id}
    response = self.connection.request(action=('/servers/%s/ips/%s/firewall_policy' % (server_id, ip_id)), data=body, method='POST')
    return self._to_node(response.object)
