Oracle Linux 8 support and fixes (#6198)

* Add oraclelinux8 and disable firewalld

Add oraclelinux8 image and disable firewalld on oraclelinux VMs

* Fix Oracle Linux repositories

As documented in: http://yum.oracle.com/getting-started.html#installing-software-from-oracle-linux-yum-server
public-yum-ol7.repo was deprecated on release 7.6. Some repos were integrated into oracle-linux-ol7.repo (i.e.: ol7_latest, ol7_addons) and other are available as packages (epel). This also adds support for oraclelinux8

* Fix to use ansible_distribution_version

Instead of ansible_distribution_major_version

* Update README.md