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

README.md

@@ -104,11 +104,11 @@ vagrant up
 - **Container Linux by CoreOS**
 - **Debian** Buster, Jessie, Stretch, Wheezy
 - **Ubuntu** 16.04, 18.04
-- **CentOS/RHEL** 7, 8 (experimental: see [centos 8 notes](docs/centos8.md)
+- **CentOS/RHEL** 7, 8 (experimental: see [centos 8 notes](docs/centos8.md))
 - **Fedora** 30, 31
 - **Fedora CoreOS** (experimental: see [fcos Note](docs/fcos.md))
 - **openSUSE** Leap 42.3/Tumbleweed
-- **Oracle Linux** 7
+- **Oracle Linux** 7, 8 (experimental: [centos 8 notes](docs/centos8.md) apply)
 
 Note: Upstart/SysV init based OS types are not supported.