Refactor mysql module

Goal is to avoid undefined variables (caught with strict_variables) and
better use class parameters to avoid useless multiplication of similar
classes.
merge master functionality into server class
merge slave functionality into server class
mysql::server: fix replication params
make 'default' performance not do any config

This way, configuration parameters can be overridden without trouble from
outside of the class.
manage perf config in one class + allow config overriding
Refactor predefined config handling
Merge pull request #60 from ckaenzig/mysql-strict-variables-refactoring

Mysql strict variables refactoring