Bluetooth: Mesh: Shell: Add cfg opt for health srv

Adds Kconfig option for the Mesh Shell Health server.

The shell implementation of Health Server test commands is
dependent on the user adding and initializing the shell Health
Server instance in their application to make the commands functional.
To prevent any confussion about how these commands work, a Kconfig
option has been added to allow conditional compilation of these
commands. In this way the user must actively enable this feature, making
the conditional requirements more clear.

Signed-off-by: Anders Storrø <anders.storro@nordicsemi.no>