Add managed host check for CNAME targets during create and update changes

[mikenairn]

What

Update the plan logic to check that all CNAME target values exist, or will exist, if it is determined that the target should be part of the managed record set i.e. it has DNSRecord.rootHost as a suffix. If the target does not exist, an error should be added to the plan errors collection so the dnsrecord controller can see it and add it as an error in the DNSRecord status.

Why

It is expected that any records using a rootHost are entirely managed by the dns operator and therefore should always have a valid chain of records. The endpoints in the local record might not contain all the steps in that chain, but the zone in which they are being published should, or should eventually, and until they do each record should report an error.

This is required for the likes of default geos where not all DNSRecords will be creating a default geo endpoint, but others might be referring to it in an endpoints target.