packaging: setup: engine fqdn validation

- Moved FQDN customization in its own plugin.
- Added FQDN validation, requiring the engine host name
  to be resolvable (additional reverse resolve check can
  be enabled through environment if needed).

Change-Id: Ic4724d9257506d67c663a0a3f312e80ebb04f42c
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>