Add IPv6 records for bastion hosts

This requires https://gerrit.wikimedia.org/r/#/c/109083/ to be merged
first. Also it renders https://gerrit.wikimedia.org/r/#/c/108027
obsolete (the latter should be abandoned, this commit is more complete)

Change-Id: Ia62eb2c682f8979a6b840fe972e5f64c55311742