Allow different kayobe container image base images

This change adds an argument to change the base image when building
a kayobe container image. This is required for the Zed release
of Openstack because there is no support for CentOS 8.

docker/kayobe/Dockerfile

@@ -1,6 +1,7 @@
 # syntax=docker/dockerfile:1.2
 
-FROM quay.io/centos/centos:stream8
+ARG BASE_IMAGE="quay.io/centos/centos:stream8"
+FROM ${BASE_IMAGE}
 MAINTAINER "Will Szumski" <[email protected]>
 
 # Unclear at this time if different environments will change