OHAI-587: Rackspace plugin rescues Errno::ENOENT if xenstore-* utils are not found

The Rackspace plugin attempts to use `xenstore-ls` and `xenstore-read` to extract cloud server information regarding the region and networking. This does not exist on FirstGen servers, so an `Errno::ENOENT` (file not found) exception is thrown. It seems the ideal exception might be `Ohai::Exceptions::Exec`, but usage of the `Mixlib::ShellOut` library returns the uncaught `Errno::ENOENT`.

https://tickets.opscode.com/browse/OHAI-587
It looks like `ohai/mixin/command` is the only place where `Ohai::Exceptions::Exec` is raised. Since the Rackspace plugin now uses `shell_out` I think we can remove the rescue from `Ohai::Exceptions::Exec`.