Allow create image command to fetch manifests files from folder

As a first step for the assets integration. the create image command will need to fetch the required ztp manifest files from the cluster-manifests folder.

This will allow to:
1) Get the manifest file from the right location
2) seamlessly integrate the create image command with the create cluster-manifests one as the tasks related to assets generation are still in progress

3) Keep the create image command fully working until the assets generation will completed (users will still be able to create/edit manually the assets in the cluster-manifests folder)