It looks like the issue is still there. I faced it deploying rocky. I used the official CentOS-OpenStack-rocky.repo

openstack-tripleo-common-9.3.0-1.el7.noarch.rpm (http://mirror.centos.org/centos/7/cloud/x86_64/openstack-rocky/)

(undercloud) [stack@undercloud ~]$ openstack overcloud image build
Running ['disk-image-create', '-a', 'amd64', '-o', './overcloud-full', '-t', 'qcow2', '-p', 'python-psutil,python-debtcollector,plotnetcfg,sos,ntp,device-mapper-multipath,openstack-heat-agents,os-net-config,jq,dbus-python,yum-plugin-priorities', '--min-tmpfs=7', '--mkfs-options', '-s size=4096', 'centos7', 'baremetal', 'dhcp-all-interfaces', 'openvswitch', 'overcloud-agent', 'overcloud-full', 'overcloud-controller', 'overcloud-compute', 'overcloud-ceph-storage', 'puppet-modules', 'stable-interface-names', 'grub2', 'element-manifest', 'dynamic-login', 'iptables', 'enable-packages-install', 'pip-and-virtualenv-override', 'dracut-regenerate', 'remove-machine-id', 'remove-resolvconf', 'openssh', 'disable-nouveau', 'selinux-permissive']
Logging output to ./overcloud-full.log
2018-09-22 12:20:06.633 | diskimage-builder version 2.16.0
2018-09-22 12:20:06.635 | Building elements: base centos7 baremetal dhcp-all-interfaces openvswitch overcloud-agent overcloud-full overcloud-controller overcloud-compute overcloud-ceph-storage puppet-modules stable-interface-names grub2 element-manifest dynamic-login iptables enable-packages-install pip-and-virtualenv-override dracut-regenerate remove-machine-id remove-resolvconf openssh disable-nouveau selinux-permissive
2018-09-22 12:20:06.790 | Traceback (most recent call last):
2018-09-22 12:20:06.790 |   File "/bin/element-info", line 10, in <module>
2018-09-22 12:20:06.790 |     sys.exit(main())
2018-09-22 12:20:06.790 |   File "/usr/lib/python2.7/site-packages/diskimage_builder/element_dependencies.py", line 342, in main
2018-09-22 12:20:06.791 |     elements = _get_elements(args.elements)
2018-09-22 12:20:06.791 |   File "/usr/lib/python2.7/site-packages/diskimage_builder/element_dependencies.py", line 253, in _get_elements
2018-09-22 12:20:06.791 |     return _expand_element_dependencies(elements, all_elements)
2018-09-22 12:20:06.791 |   File "/usr/lib/python2.7/site-packages/diskimage_builder/element_dependencies.py", line 148, in _expand_element_dependencies
2018-09-22 12:20:06.791 |     raise MissingElementException("Element '%s' not found" % element)
2018-09-22 12:20:06.791 | diskimage_builder.element_dependencies.MissingElementException: Element 'disable-nouveau' not found
Command '['disk-image-create', '-a', 'amd64', '-o', './overcloud-full', '-t', 'qcow2', '-p', 'python-psutil,python-debtcollector,plotnetcfg,sos,ntp,device-mapper-multipath,openstack-heat-agents,os-net-config,jq,dbus-python,yum-plugin-priorities', '--min-tmpfs=7', '--mkfs-options', '-s size=4096', 'centos7', 'baremetal', 'dhcp-all-interfaces', 'openvswitch', 'overcloud-agent', 'overcloud-full', 'overcloud-controller', 'overcloud-compute', 'overcloud-ceph-storage', 'puppet-modules', 'stable-interface-names', 'grub2', 'element-manifest', 'dynamic-login', 'iptables', 'enable-packages-install', 'pip-and-virtualenv-override', 'dracut-regenerate', 'remove-machine-id', 'remove-resolvconf', 'openssh', 'disable-nouveau', 'selinux-permissive']' returned non-zero exit status 1

I was able to proceed only when I commented out the "- disable-nouveau" line in /usr/share/openstack-tripleo-common/image-yaml/overcloud-images.yaml