CAM5 UW PBL: make hard-coded tunable parameters namelist options

From review of https://github.com/ESCOMP/atmospheric_physics/pull/370

@nusbaume originally wrote:
> We can deal with this in a future PR, but if if these are all tunable parameters then it seems like it would be better if they were namelist variables instead of hardcoded parameters. Maybe worth making a "future work" issue?
