Use dinamic inventory for Vagrant.

Removes inventory file for debian6 (squeeze) VM on Vagrantfile favouring
the dinamic inventory manipulation of inventory lready provided by
Vagrant.
Adds centos6 VM to Vagrantfile.

Allowing better tests on RedHat family OSes. This new machine has an
unique internal IP and is neither the primary nor the auto start VM used
by Vagrant (Debian Squeeze).
Add `my.cnf` !includedir configuration.

Enables configuration files to be added directly from another directory,
as used by Debian for `mysql-server` package.

Re-organizes README sections for role variables, dividing them into
sections.
Different `my.cnf` files through configuration.

Exposes through `mysql_conf_file` variable the absolute path to the
configuration file, which differs from OS to OS. The default value is
the one used previously (RedHat), adds the variable declaration with the
right path into `vars/Debian.yml` file.
Enable vagrant-cachier plugin.
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.
Merge pull request #4 from augustohp/mysqld_multi

Multiple MySQL instances