packaging: setup: added postinstall file creation

Added a plugin for engine-setup-2 that creates a
postinstall file with answered questions so they
can be avoided on second run / upgrade.

Change-Id: I3516d6b9b29d50844835bb67c0a4a3f5485aa21f
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>