Quickstart fails if Virtual BMCs have already been created

The quickstart-extras task that creates Virtual BMCs will fail if they already exist:

TASK [undercloud-deploy : Create the Virtual BMCs] *****************************
task path: /home/ooo/.quickstart/usr/local/share/ansible/roles/undercloud-deploy/tasks/configure-vbmc.yml:61
Friday 21 July 2017 11:51:29 -0400 (0:00:00.122) 0:26:42.667 ***********
failed: [undercloud] (item={u'flavor': u'control', u'virtualbmc_port': 6230, u'name': u'control_0'}) => {"changed": false, "cmd": ["vbmc", "add", "control_0", "--
port", "6230", "--libvirt-uri", "qemu+ssh://stack@192.168.23.1/session?socket=/run/user/1002/libvirt/libvirt-sock&keyfile=/root/.ssh/id_rsa_virt_power&no_verify=1
&no_tty=1"], "delta": "0:00:01.112262", "end": "2017-07-21 15:51:41.162018", "failed": true, "item": {"flavor": "control", "name": "control_0", "virtualbmc_port":
 6230}, "rc": 1, "start": "2017-07-21 15:51:40.049756", "stderr": "Domain control_0 already exists", "stdout": "", "stdout_lines": [], "warnings": []}

This makes it difficult to re-runing quickstart without rebuilding or patching the undercloud node.