migrations: migrate new cluster-domain setting

[etungsten]

Add a migration for migrating the new cluster-domain setting

Issue number:
N/A

Description of changes:

Author: Erikson Tung <etung@amazon.com>
Date:   Thu Aug 13 09:32:04 2020 -0700

    migrations: add `cluster-domain` setting
    
    Add a migration for migrating the new `cluster-domain` setting

Testing done:
With a local datastore:
Backward migration from 0.5.0 datastore

$ VARIANT=aws-k8s-1.17 cargo run -- --source-datastore ~/thar/testing/ds/current --target-datastore ~/thar/testing/ds/next-backward --backward
...
Found no settings.kubernetes.cluster-domain to remove
Removed settings.kubernetes.cluster-domain, which was set to '"cluster.local"'
etung in migrations/v0.5.0/add-cluster-domain on 🌱 develop [⇡$!?] via 🦀 v1.45.2 took 9s

Forward migration from there:

$ VARIANT=aws-k8s-1.17 cargo run -- --source-datastore ~/thar/testing/ds/next-backward --target-datastore ~/thar/testing/ds/next-forward --forward
    Finished dev [unoptimized + debuginfo] target(s) in 0.17s
     Running `/home/ANT.AMAZON.COM/etung/thar/PRIVATE-thar/sources/target/debug/add-cluster-domain --source-datastore /home/ANT.AMAZON.COM/etung/thar/testing/ds/next-backward --target-datastore /home/ANT.AMAZON.COM/etung/thar/testing/ds/next-forward --forward`
AddSettingsMigration(["settings.kubernetes.cluster-domain"]) has no work to do on upgrade.
AddSettingsMigration(["settings.kubernetes.cluster-domain"]) has no work to do on upgrade.

No work done as expected on upgrade.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.