Bluetooth: Mesh: fix access to uninitialized memory

Running bsim with Valgrind highlighted a couple of
places where read access to uninitialized memory happens.
Config client rewrites publication parameters with not
initialized uuid pointer. Bitfields those are allocated on
the stack keeps not used area uninitialized too.
Uninitialized area is stored in the persistent memory.
PR fixes that.

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