make salt minion id more configurable

[do3meli]

per default the salt minion does create the minion_id file with the short hostname if it does not exist on its first startup. in some environments the salt minion id is required to be a fully qualified domain name. therefore i recommend to have a salt minion cloud-config parameter that allows to be set to true/false and based on the value takes the FQDN or the shortname and writes it to the minion_id file. alternatively the minion id could also be fully configurable. meaning: the whole config string is taken and written to the minion_id file.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3135