Add events.mechanism parameter to fpm config

It should not be set unless needed so it's not on default vars.
Merge pull request #16 from hernandesbsousa/add_events_mechanism_to_fpm

Add `events.mechanism` parameter to `php-fpm.ini`
Smaller VM hostnames and avoid IP conflicts.

If all VMs present on Vagrantfile were running there would be IP
conflicts between internal network, each machine now has its own IP
address.

The prefix `php-role` was removed fromt he host name as I see them
redundant to the project. The VM name still the same though, it may be
usefull to indentify the VM on the list of all VMs running under a
virtualization provider (virtualbox-only supported).
Support php_versions configuration.

Enabling specifying a different version of PHP form the one that
defaults to the distribution. Not all versions are supported by every
distribution, a version compatibility matrix is available on the README.

Each version and lsb distribution has its own configuration file, if a
configuration file is not present the provisioning will fail (because
the file doesn't exist).

This adds a simple shell script that combined with the changes on
Vagrantfile (a machine for each supported PHP version and distribution)
can be considered as our test suite, since our CI does not have multiple
OSes and the ones I tested were crap.