Fail fast if not ready
Perform validation at action execution time. 

By default Vagrant performs config validation for ALL plugins at EVERY command execution. This would cause simple commands like `vagrant ssh` or `vagrant status` to fail if the `adam.provision_url` is not configured in a `Vagrantfile` for example.
Only run provision script job if in config

Should fix #7, as now it won't run if config value is empty.