When no `--port` is specified, TensorBoard should try to find one

[wchargin]

Currently, when no port is specified, TensorBoard defaults to 6006. If
binding to 6006 fails, TensorBoard dies. Instead it should be the case
that TensorBoard tries harder to find a port, probably incrementally
(6007, 6008, …). This is pretty common among tools that bind to
local ports (e.g., webpack-dev-server or the closure_rules web server).

If an explicit port is specified like --port 6006 and the port is not
available, then TensorBoard should die.