The userdata template preseed_autoinstall_cloud_init defines the installer configuration for a new Ubuntu host. This includes also an initial package manager setup which configures a repository for apt.
As of now, the URL http://archive.ubuntu.com/ubuntu is set statically. This leads to an error if the host cannot reach this URL during the initial installer setup. This is usually the case if a host is supposed to be deployed offline and should not have access to the Internet.

We need an option to enable offline installation for Ubuntu host deployment. This could be achieved by assigning a mirror dynamically.