tests: Bluetooth: Mesh: create reduced legacy configuration

mesh_shell application has been included recently some set new
abilities that added more functionality for testing.
That caused the application cannot fit in some supported plarforms
like qemu_cortex_m3. The reduced legacy configuration has been
added to fit stack into it. This platform is used in CI.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>