aks-preview 19.0.0b21

• az aks nodepool get-rollback-versions: New command to get available rollback versions for an agent pool.
• az aks nodepool rollback: New command to rollback an agent pool to the most recently used configuration (N-1).

SHA256 hashes of the release artifacts:

304d55dca4784d4934183b68d76623c59c02eeb80cbf6d21b5aa1f06c377d0a5 aks_preview-19.0.0b21-py2.py3-none-any.whl