Adjusted playable sides for LoW.

The changes are not final:
"Costly Revenge" and "Elvish Assasins" scenario sides need specific
adjustments for MP. E.g. gold, placement.
"Elves' Last Stand" and "The Chief Must Die" have different number of
sides than other scenarios in the same chapters.

In addition, mp::connect_engine's side controller set up has been
slightly adjusted.