[config:generate] Make the installed PHP version configurable

[opdavies]

With the Drupal VM 3.0.0 release, the default value of php_version changed to 7.0.

The other supported versions are 5.5 and 5.6.

These options should be presented to the user during the interaction stage to allow them to easily change the PHP version.

[opdavies]

Re-opening. The commit has been reverted, as there are more variables that need to be added to get a different version of PHP installed so this does nothing by itself. See http://docs.drupalvm.com/en/latest/other/php-56/.

[opdavies]

This could be achieved once custom variables are added in #13, as well project level defaults in #43 as the package names will be different per guest OS/project.

[opdavies]

Following geerlingguy/drupal-vm#714, as this would be better if it was handled upstream.

[opdavies]

This is now configurable within config.yml (see https://github.com/geerlingguy/drupal-vm/blob/master/default.config.yml#L239-L241), so will be fixed within #44.