add support to handle various mysql vendors

Since I use cookbook/database but also I want to use cookbook/percona instead of mysql I had unresolvable conflict in mysql-chef_gem. This small patch add support to 'vendored' version of mysql. Tested with percona and mariadb. I am still no sure how to handle dependency from these cookbooks so I've left as it is.

To use mysql-chef_gem with percona one need to define node attribute e.g in recipe:.

    node.set['mysql-chef-gem']['vendor'] = 'percona'