Undercloud installation fails when fetching Ceph images

Issue

• Tripleo container image pull fails with

# tail -f /var/log/tripleo-container-image-prepare.log
2020-01-27 01:56:15,729 538088 ERROR root [  ] Image prepare failed: HTTPSConnectionPool(host='docker-registry.upshift.redhat.com', port=443): Max retries exceeded with url: /v2/ (Caused by NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7fc870566898>: Failed to establish a new connection: 
[Errno -2] Name or service not known',))
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/urllib3/connection.py", line 159, in _new_conn
    (self._dns_host, self.port), self.timeout, **extra_kw)
  File "/usr/lib/python3.6/site-packages/urllib3/util/connection.py", line 57, in create_connection
    for res in socket.getaddrinfo(host, port, family, socket.SOCK_STREAM):
  File "/usr/lib64/python3.6/socket.py", line 745, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno -2] Name or service not known