Fix "options are only allowed before install arguments" error.

Looks like creating the salt bootstrap script option string has been changed a little here. I believe the original plugin had it right. Hence, this reverts feb748f488 and 7cd7077467. 

`bootstrap_options` is meant for flag options such as `-D`. This is demonstrated in the example Vagrantfile from the original plugin: https://github.com/saltstack/salty-vagrant/blob/develop/example/complete/Vagrantfile. Flag options must be appended to the options string before install args such as `install_type` and `install_args`.

Not sure, but I'm guessing 7cd7077467 was created because @ticosax might not have been aware of `install_type` and `install_args` and was trying to put these *arguments* where the *options* belong.