R53_ALIAS records don't support TTL

[jtdoepke]

Describe the bug Per the AWS docs, R53_ALIAS records don't have their own TTL. Instead, they use the TTL of whatever record they are an alias to. If you set a TTL on a R53_ALIAS record that's different than the TTL of the aliased record, dnscontrol will see that record as perpetually out of sync.

To Reproduce

D("example.org", REG_MY_PROVIDER, DnsProvider("ROUTE53"),
    , DefaultTTL("10m")
    , A("foo", "1.2.3.4", TTL("5m"))
    , R53_ALIAS("bar", "A", "foo") // dnscontrol will continuously try to change the TTL of this to 10m.
)

Expected behavior I think it would make sense for R53_ALIAS records to ignore DefaultTTL() and TTL(). In the case of TTL(), it should probably also print a warning that it's unsupported.

DNS Provider

• AWS Route 53

Additional context

[tlimoncelli]

CC @tresni

Agreed. We should either print a warning if there is a mismatch, or copy the TTL from the underlying record, or rework the diff2 module to know to ignore TTL settings for certain record types. My preference is either of the first two.

[misilot]

R53_ALIAS('foo', 'A', 'lib-lb-502.us-east-1.elb.amazonaws.com.', R53_ZONE('ZONEID')),

Will try setting the TTL from our default back to 300 seconds, and when you look in the R53 console, TTL isn't an option that can be set.

[tlimoncelli]

A more simple solution is to update the ROUTE53 doc to explain the situation and recommend how to fix it. We can code our way out of every problem. @misilot: would you please provide a draft paragraph for the docs?