Fixing an issue where trying to install docker-ce-18.09 on rhel7 nodes (or...

Fixing an issue where trying to install docker-ce-18.09 on rhel7 nodes (or potentially centos 7) without an enabled extras repo the installation will fail because container-selinux >= 2.9 is required. The check for container-selinux upfront should obviate the need for adding an extras repo if the node is able to find it from another source. (#4161)