Add zone delegation for autojoin.measurement-lab.org.

[robertodauria]

This adds the autojoin zone delegations for autojoin.measurement-lab.org (#326 and #325 are previous PRs adding zone delegation for sandbox/staging).

See here for the zone's configuration.

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This change is 

[nkinkade]

with one small cleanup request.

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @robertodauria)

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formats/v2/zones/measurement-lab.org.zone.jsonnet line 46 at r1 (raw file):

                     IN     NS      ns-cloud-a3.googledomains.com.
                     IN     NS      ns-cloud-a4.googledomains.com.
    ; Delegate mlab-oti subdomain to staging Cloud DNS servers.

While you are in here, could you change this comment to read correctly, should be mlab-oti to production (not staging) servers.