packaging: setup: clean up answer files writing

Always write one to /var/lib/ovirt-hosted-engine-setup/answers .

If user asked for one with --generate-answer, always write there.

Write to /etc only at end of successful setup.

Support relative paths for answer files.

Bug-Url: https://bugzilla.redhat.com/1151366
Bug-Url: https://bugzilla.redhat.com/1121057
Related-To: https://bugzilla.redhat.com/1025313
Change-Id: Icb28356a0ee38b325c53ec631b21b912e044d2a9
Signed-off-by: Yedidyah Bar David <didi@redhat.com>
(cherry picked from commit fcb1d7f0322c108719cd48146b11d6780c945031)