weapons.yaml update for CNC

See https://github.com/OpenRA/OpenRA/pull/2655 for details and rationales.
scaled back tank firing rate a bit

Originally I increased the tank firing rate by 2x, but I think 1.5x makes more sense. (Now fairly comparable to RA values). Going to leave it here.
Reduced Artillery damage vs. wood from 75% to 50%.
Slight tweak to recon bike damage 
Slight tweak to Stealth Tank damage vs. heavy armor
fixed APC gun mistkae -- now shoots at air too
Merge pull request #2721 from psydev/patch-1

weapons.yaml update for CNC