charms: Support `preseed` bootstrapping

[gabrielmougard]

Using the initial preseed work #144, we could introduce a new preseed charm configuration key during the deploy phase like:

juju deploy ./microcloud_ubuntu-22.04-amd64.charm -n 3 --to 0,1,2 --config preseed=<PATH_YAML_PRESEED_FILE>

To deploy the microcloud units according to the preseed rules.