ORT syncds bug - Parent holds up child running syncds if TO has multiple CDNs

Based on: https://github.com/Comcast/traffic_control/issues/1380

By: jpappa200
A parent from a different CDN can hold up a child from running syncds if Traffic Ops has multiple CDN's configured.

https://<to_host>/update/ looks at all parents associated with the cache group and doesn't check which CDN it's associated with.

Comment by jeffmart:
Is this hold up indefinite?
or
does syncds complete on the child after all parents for all CDNs in that cache group are updated?
I am assuming https://github.com/Comcast/traffic_control/pull/2 but want to make sure

knutsel :
so we have a issue somewhere to take regex_revalidate out of the ort/syncds distribution mechanism, and scp (or ansible dist) it directly on update; this would remove the "mids have to complete ort syncds before edges" requirement, and things should get a lot better when we do that...