Add versionadded for varnish module

Also did some cleanup and made it conform to the way Salt does things
Add varnish module to docs
Remove redundant function
Fix refs to removed function _detect_os()

Also convert some double-quotes to single-quotes and fix the purge
function. It was never running the command that it built, and it should
have been returning a boolean.
Merge pull request #13167 from terminalmage/varnish-cleanup

Cleanup new varnish module and add to docs