Drop dependency on opscode/mysql cookbook

The dependency on the mysql cookbook is getting out of date, the new cookbook is completely different, and this cookbook doesn't really use any of it anyway: we install our own packages, set up our own service, etc.

It might be good to eventually bring it back inline if some of the mysql cookbook can be reused (maybe subclass their libraries, or agree on configurable params) since a lot of it does the same things.

For now, however, this eliminates any conflicts with other cookbook versions.

Note, I've updated the default version to the latest (5.6) for new installations, set by a new percona.version attribute. This change warrants a version bump in the cookbook metadata.