defaults, migrations: update default admin-container version migration

[etungsten]

Issue number:
N/A

Description of changes:

Author: Erikson Tung <[email protected]>
Date:   Thu Aug 13 13:19:53 2020 -0700

    defaults: update default admin container version to v0.5.2
    
    Changes default admin container version from v0.5.1 to v0.5.2

Author: Erikson Tung <[email protected]>
Date:   Thu Aug 13 13:17:59 2020 -0700

    migrations: update default admin-container migration to migrate to v0.5.2
    
    Updates default admin container version migration to migrate to v0.5.2 instead of v0.5.1

Testing done:
From a local datastore:

Forward migration:

$ VARIANT=aws-k8s-1.17 cargo run -- --source-datastore ~/thar/testing/ds/current --target-datastore ~/thar/testing/ds/next --forward
 ...
Updating template and value of 'settings.host-containers.admin.source' on upgrade
Changing template of 'settings.host-containers.admin.source' from '328549459982.dkr.ecr.{{ settings.aws.region }}.amazonaws.com/bottlerocket-admin:v0.5.0' to '328549459982.dkr.ecr.{{ settings.aws.region }}.amazonaws.com/bottlerocket-admin:v0.5.2'
Changing value of 'settings.host-containers.admin.source' from '328549459982.dkr.ecr.us-west-2.amazonaws.com/bottlerocket-admin:v0.5.0' to '328549459982.dkr.ecr.us-west-2.amazonaws.com/bottlerocket-admin:v0.5.2'
Found no 'settings.host-containers.admin.source' to change on upgrade

Backwards migration:

$ VARIANT=aws-k8s-1.17 cargo run -- --source-datastore ~/thar/testing/ds/next --target-datastore ~/thar/testing/ds/next-backwards --backward
    Finished dev [unoptimized + debuginfo] target(s) in 0.45s
 ...
Updating template and value of 'settings.host-containers.admin.source' on downgrade
Changing template of 'settings.host-containers.admin.source' from '328549459982.dkr.ecr.{{ settings.aws.region }}.amazonaws.com/bottlerocket-admin:v0.5.2' to '328549459982.dkr.ecr.{{ settings.aws.region }}.amazonaws.com/bottlerocket-admin:v0.5.0'
Changing value of 'settings.host-containers.admin.source' from '328549459982.dkr.ecr.us-west-2.amazonaws.com/bottlerocket-admin:v0.5.2' to '328549459982.dkr.ecr.us-west-2.amazonaws.com/bottlerocket-admin:v0.5.0'
Found no 'settings.host-containers.admin.source' to change on downgrade

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.