Bluetooth: Mesh: Shell: Split out mdl shell feat

This commit splits out the model related commands from mesh shell to
separate shell modules. The purpose of this alteration is to make model
commands more flexible by letting the application itself define the
model instances.

- Move Health Client commands to separate shell module.
- Move Config Client commands to separate shell module.
- Remove Health Client instance from shell.c.
- Add Health Client instance to mesh_shell test application.
- Add util library for Mesh shell.
- Some refactoring of shell menu layout.
- Update Mesh shell documentation.

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