shell: mqtt: use topic levels

Change topic from <device_id>_rx (and tx) to <device_id>/sh/rx.

This allows use of wildcards. E.g. subscribe to all devices "+/sh/tx".

Level "sh" is added to the topic to make it less generic and prevent
potential clashes with other topics.

The topic part after <device_id> is configurable via Kconfig.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>