Support multiple mysql instances on same host.

Using `mysqld_multi` and different groups of configuration into `my.cnf`
we are able to use how many instances we want into a single host.

Divides some tasks into `simple.yml` and `multi`.yml` task scripts,
which get run if you define multiple instances using `mysql_multi`
variable or not.

Using multiple instances disables completly single instances on the
host, and a rollback (from multiple instances) to single instances will
probably cause problems.

Creates and enables an init script to boot up multiple instances on the
host.