Added execute client handler method.

    @listed

    def resume(self, query):

        self._vm_action(query, 'resume_vm')

    @pure

    @listed

    def execute(self, query, command):

        query += '&hv'

        query = TqlQuery(query)

        hypervisors = query.filter(list(self._server.iterrole('hypervisor')),

                                   key=lambda o,n: o.get_tags([n]).get(n))

        outputs = []

        for hv in hypervisors:

            outputs.append(hv.connection.call('execute_command', command))

        return outputs

class AuthenticationError(Exception):

    pass