Support setting errorcachingminttl on cdns

[bengerman13]

We should support setting ErrorCachingMinTTL on cdn instances. Once we do this, we need to update the migrator to parse and update the config (This should also support distributions where ResponseCode is empty string and ErrorCode is an int)

When we write this, we need to write an alembic migration to backport the existing mincachingttl to distributions that have it configured.

[markdboyd]

@bengerman13 Was this a customer request? If not, can we close and deprioritize pending a customer request?

[bengerman13]

I think this was thought to be broadly interesting to customers, especially Pages users, to prevent cache poisoning