linkerd/linkerd2-proxy#213 introduces a new control client to an identity service. This change breaks the outbound_http{1,2}_l5d_server_id_l5d_client_id tests in the discovery test module, since we can't actually instrument tests that provide with the proxy with identity yet.

As we have done for the destination client, we should add a mock identity service that can be instrumented to drive tests. We should at least re-enable the above tests; though we should probably add dditional tests that ensure the proxy refreshes identity as expected.