Fix top.sls compilation (#12483) so it matches the behaviour description in documentation section 21.24.15.3 exactly

Proposed fix for #12483

I found best way to debug was at end of merge_top put in a log.info that calls json.dumps( top ) (needed to import json) and it gave a nice pretty output to test things were working OK. Not sure if it worth putting that back in and keeping as a debugging option?

This **changes** the behaviour of top.sls compilation so it behaves exactly as the documentation describes at:
21.24.15.3. HOW TOP FILES ARE COMPILED
http://docs.saltstack.com/en/latest/ref/states/top.html#how-top-files-are-compiled

Specifcally, 'base' is king, then a top.sls in X environment is king of X environment (assuming 'base' isn't already kind). Then it's alphabetical order.

Currently, it's purely alphabetical order so nothing like what the above documentation states.