Don't overwrite the entirety of kdump.conf when we save

Be smarter about writing kdump.conf -- only clear and set values
which may be set by the TUI so we don't stomp on other people.

Change-Id: Ic53bab0b3a5602ac71028db5835a69005ac85a0c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1122923
Signed-off-by: Ryan Barry <rbarry@redhat.com>
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>