Chef 4994: knife cookbook site share fails on windows

Full ticket details here: https://tickets.opscode.com/browse/CHEF-4994

cookbook_site_share was using the deprecated Chef::Mixin::Command.  Moving it over to Chef::Mixin::ShellOut clears up the issue.

Tested by manually uploading a cookbook from Mac & Windows as well as running unit and functional tests on Mac OS.
I had it in that format initially, but spec wasn't mocking shell_out! then.  It would happily pretend it did and then many tests would fail in a way that showed it clearly wasn't.  I can dig into that more though.  It was bothering me anyway.