Make cron provider work when Chef is run as non-root user.

Also, move it onto Chef::Mixin::ShellOut.

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>
If you want to help the user out it'd probably be better to decorate the exception that you get back from shell_out to mention "hey, this blew up and it looks like you tried to change users when not running as root, that's likely your issue"