Proposal: enhanced mysql pillar

This isn't quite ready for merging, I think I still need to reconsider how it handles paramatization and the docs might need further work, but it's at a point where it's ready for a PR and some feedback.

If you'll pardon the pun, the aim of this exercise is to add depth to the MySQL pillar.  The code is designed such that it could be moved to a separate file and a PGSQL pillar written with fairly minimal effort.