Update Salt's virtualenv settings

[c17r]

1) no_site_packages option has been deprecated for system_site_packages 2) if the requirements file has references to other requirement files on the file system, Salt will break unless no_chown option is given which prevents Salt from copying the requirements file to /tmp/ before running.

[audreyfeldroy]

:+1: This was very helpful, thanks @sauerc

[wunki]

Thanks!