Require mysql::client in mysql type-classes instead of mysql::server

Otherwise we'd mix "include mysql::server" and "class {'mysql::server':}" in some cases (when using exported resources), which will result in:
 Duplicate declaration: Class[Mysql::Server] is already declared; cannot redeclare at /etc/puppet/modules/mysql/manifests/server/instance.pp:6
Merge pull request #720 from njam/issue-720

Fix mysql::server duplicate declaration