[train][goal] set default dns for IPv6 env
[train][goal] set default dns for IPv6 env

'MURANO_DEFAULT_DNS' is hard coded for IPv4 default dns.
For IPv6 env, we need to set it to ipv6 default dns.

Change-Id: Ie01b9ccc3ce1ca538142a49dc107f742252e46b8
Story: #2005477
Task: #35907

openstack/murano,openstack/murano