make salt minion id more configurable

This bug was originally filed in Launchpad as [LP: #1755204](https://bugs.launchpad.net/cloud-init/+bug/1755204)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2018-03-12T15:15:26.961508+00:00

date_fix_committed = None

date_fix_released = None

id = 1755204

importance = low

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1755204

milestone = None

owner = d-info-e

owner_name = do3meli

private = False

status = confirmed

submitter = d-info-e

submitter_name = do3meli

tags = []

duplicates = []

_Launchpad user **do3meli(d-info-e)** wrote on 2018-03-12T15:15:26.961508+00:00_

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.