FYI, here is an interim workaround for getting CentOS 8 on MAAS:
$ wget https://people.canonical.com/~ltrager/centos8-1.0.2-7-gac521bb.tar.gz
$ maas $PROFILE boot-resources create name='centos/8-packer' title='CentOS 8 Packer' architecture='amd64/generic' filetype='tgz' content@=centos8-1.0.2-7-gac521bb.tar.gz