Capability with Parallels provider

I've added capability with Parallels provider (['vagrant-parallels'](https://github.com/yshahin/vagrant-parallels)) to your plugin. I tested it by myself and everything is working fine. Also, I've fixed a help info for `sandbox` command  

@jedi4ever, I can help you with supporting Parallels provider in `sahara` plugin, if there will be some issues reported.