Fedora CoreOS will replace Fedora Atomic being the next generation
container OS. So it would be nice to support Fedora CoreOS in Heat.
In Fedora CoreOS, the cloud-init will be replaced with Ignition[1],
so the changes proposed in this patch are mainly focusing on how to
support Ignition when using Heat SOFTWARE_CONFIG with Ignition.
Task: 36671
Story: 2006566
Change-Id: I11df2431634de7d8b584b1a2ac733d43959e34fc
(cherry picked from commit f6249c0110)