Failing setup module if selinux is enabled but no libselinux-python is installed

This change will fail the setup module if it detects Selinux on the target node, but that libselinux-python package isn't installed, and so that prevents future issues when playbooks/tasks are played on a selinux enabled node but that no selinux context can be applied (for example the authorized_keys module for correct selinux context on ~/.ssh/authorized_keys). Don't know if that the best way to handle it, but I'd prefer myself seeing Ansible alerting the users for a missing package, than playbooks all marked OK in green when it doesn't work as expected in the background. It's to be noted that CentOS/RHEL 6.x when installed in "minimal" way (aka only @core packages group) don't have libselinux-python rpm package installed, and that's how i discovered the "issue" ....