'Add a hard disk to the server
:param server_id: Id of the server
:rtype: ``str``
:param size: Size of the new disk
:rtype: ``str``
:param is_main: Indicates if the disk is going to be the boot disk
:rtype: ``boolean``
:return:    Instance of class ``Node``
:rtype:     :class: `Node`'

def ex_add_hdd(self, server_id, size, is_main):

    body = {'size': size, 'is_main': is_main}
    response = self.connection.request(action=('servers/%s/hardware/hdds' % server_id), data=body, method='POST')
    return self._to_node(response.object)
