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>
You can probably keep this all as an array and avoid the whitespace stuff:

```
crontab_cmd = [ "crontab" ]
crontab_cmd << "-u #{@new_resource.user}" if @new_resource.user
crontab_cmd += arguments
shell_out(crontab_cmd.join(' '), options)
```