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.
You could add "include Chef::Mixin::ShellOut" in the class so you can call #shell_out! I don't know if it saves many characters in this case yet, but it is the pattern I'm used to seeing.