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

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:
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`.
Merge pull request #3005 from ajw0100/fix-salt-options-error

provisioners/salt: Fix "options are only allowed before install arguments" error.