Error in plugin installation cause an infinite loop

[CristianCantoro]

Related to issue https://github.com/hugsy/modern.ie-vagrant/issues/4, since the plugin installation is setting _retry=true if something goes wrong during the install (and thus the plugin is not installed), the script will continue to try the installation and re-executing itself.

Given how vagrant this means that any command, including for example vagrant status will give rise to the infinite loop.