[DM] GKE schema fixes (#594)
[DM] GKE schema fixes (#594)

* Fix shieldedInstanceConfig and workloadMetadataConfig schemas

* Network providers are singular

* Node metadata is singular

* Add containerd variants
GoogleCloudPlatform/cloud-foundation-toolkit,GoogleCloudPlatform/cloud-foundation-toolkit,GoogleCloudPlatform/cloud-foundation-toolkit,GoogleCloudPlatform/cloud-foundation-toolkit,GoogleCloudPlatform/cloud-foundation-toolkit