🌱 Merge /etc/elemental/config.yaml with the user cloud-config

[mudler]

Is your feature request related to a problem? Please describe.
Currently some upgrade/install configurations are expected to be present in /etc/elemental/config.yaml. So that means if for instance we want to change the default size of the active image ( see also #1120 ) we need to write to that file during the cloud-init config stages or customize the image.

Describe the solution you'd like
The cloud config file should be merged or expose fields for the configuration of such file in runtime.

Describe alternatives you've considered
N/A

Additional context
N/A