CentOS 8: Deploy CentOS 8 containers

* HAProxy is now 1.8 in CentOS 8
* Support python3 in baremetal role
* Remove support for environments without python2 installed (this could
  not have worked since we gather facts before this point)

Workarounds:

* Using CentOS 7 yum repo for Docker, with module_hotfixes

Change-Id: I30bd3d58f6224ad4c9575ba66c74deabe6895cc4
Partially-Implements: blueprint centos-rhel-8