Add feature to pass or mount user files during image qcow2ova conversion

[Amulyam24]

As of now, during image conversion we can only add commands in the template than can be run during the build. It would be good to have a feature for passing user files that are to be present in the final image after build.

example files: SSH keys

[mkumatag]

Let's implement a generic solution for temporarily mounting a directory from the local host, and then using any necessary commands in the preparation template to utilize that mount.