Works with "yum" cookbook v3.0

This change makes `percona` cookbook compatible with `yum` cookbook version 3.0, which:
* removed support for `yum_key`
* changed args for `yum_repository` 

`recipes/package_repo.rb`, `Berksfile` and `metadata.rb` have been updated.

I realize this will break some folks' cookbook dependencies, and it may be better to include in a major revision bump.  Are there any of those on the horizon?  

Thanks!