planner: add external DNS zones when needed

[davepacheco]

See RFD 459. To properly support expunging external DNS zones, we'll need the planner to make sure it provisions new ones so that all the expected external DNS IPs are covered.

[davepacheco]

I think this was done by #6581, right @plotnick?

[plotnick]

Yes, I believe this is done (modulo bugs).