A new config option `post_install_manifest_url` is added to support
installing cloud provider/vendor specific manifest after booted
the k8s cluster. It's an URL pointing to the manifest file. For
example, cloud admin can set their specific storageclass into
this file, then it will be automatically setup after created
the cluster.
Task: 35798
Story: 2006209
Change-Id: Ib5a2c5cd7970085db941f189613e175f622aea3f