Fix the database dependencies

 - Don't include MySQL recipes when not using MySQL.
 - Don't include mysql::client at all, database::mysql is sufficient.
 - Do include database::postgresql when using PostgreSQL.
 - Bump database dependency due to breaking changes in mysql-chef_gem.
 - Demote mysql and postgresql cookbooks to "suggests" as they are not
   strictly required.
Merge branch 'database_dependencies' into yakara