SELinux is touched only if SELinux is installed.

This is a change propagation from Amazon Linux support to RedHat7.
Amazon Linux AMI is removed from README.md

If the playbook is run on Amazon Linux AMI 2014.09.1 (HVM) - ami-4985b048, it is found that the playbook hangs at template module during "generate the pexpect script" task in roles/lxr/tasks/160-config_lxr.yml.  Untill the issue is resolved, Amazon Linux AMI is removed from the doc.