support backups on replicas

[michaelglass]

in order to support gh-ost's need for a MySQL binlog, we want to optionally enable 1-day backups on a replica.

[michaelglass]

whoops, wrong repo. Actually happy to push here or our fork. you tell me, @dgtized

[dgtized]

Happy to accept pull requests, would much prefer knowledge that current version is in active use, but pretty sure my version is ahead of your fork and slightly diverged. Specifically that the current version here also supports exporting a minimal IAM policy to allow stack-mitosis to run, which might be useful on your end, but on your fork it looks like you hardcoded a different timeout (which I intended to add as a flag at some point).

With regards to the requested feature, by default stack-mitosis enables backups on every replica that also has children. However, it can trivially enable backups on replicas if the original replica already has backups enabled. I believe that should be added to https://github.com/dgtized/stack-mitosis/blob/master/src/stack_mitosis/lookup.clj#L115 and then remove the special case in https://github.com/dgtized/stack-mitosis/blob/master/src/stack_mitosis/planner.clj#L58. That behavior is used now to transfer any existing modifications to any instance in the stack to any copies. If that behavior is also applied for BackupRetentionPeriod, then any replicas that have backups before the clone will have it after as well, even if they don't strictly require it for the clone operation.

Let me know how you would like to proceed.

[dgtized]

Also I think putting it in post-create-replica-attributes would also correctly copy custom values of BackupRetentionPeriod like 2, or 30. So I think that's better anyway.