Read Pillar files into OrderedDict to preserve source order

This pull request attempts to solve Enhancement #12161.

I'm very new to Salt, but It's something i'd like for my own states so I thought working on a patch was a good way to get familiar with the internals of SaltStack.

I've tested the functionality in both master-minion and masterless setup and it keeps the pillars in an OrdererdDict. 

It does not attempt to change the 'master' values, as I don't see the need and that would mean touching the code in many places.