packaging: setup: Clean up treatment of /etc/exports

Without this change, /etc/exports:
- gets overwritten on setup if there is no /etc/exports.d
- is added empty lines when edited by setup
- is not restored to previous content on cleanup

With this change:
- If there is /etc/exports.d:
-- Setup writes there a file with the iso domain path, which is removed
on cleanup (if not changed since setup)
-- Setup removes this path from /etc/exports if it's there (supposedly
added by a previous legacy Setup), and this change is not reverted on
cleanup
- If there is only /etc/exports:
-- Setup adds to it the path if it's not there already
-- Cleanup reverts this change if the line was not changed since setup

Bug-Url: https://bugzilla.redhat.com/1035511
Change-Id: I035817c048518f77fc666cc6d5212c8e46edd65d
Signed-off-by: Yedidyah Bar David <didi@redhat.com>