ceph-debug-docker: support centos 8

ceph-debug-docker: support centos 8

* default to centos:8, as we've moved to centos:8 now
* do not assume that the base image is centos:7, use centos:8 if it is
  specified.
* install python3-* packages for centos:8 and install python36-*
  packages for centos:7. as el8 is now a python3 distro, and
  centos:7 now has python36.
* s/screen/tmux/. because screen is now offered by EPEL, while tmux
  is in BaseOS.

Signed-off-by: Kefu Chai <kchai@redhat.com>