[3.0] when rebooting admin, set tx_update_reboot_needed to false

[MaximilianMeister]

otherwise we have a window after the restart, where the reboot admin button is still shown, as the event proccessor still needs time to update the grains

bsc#1099015

Signed-off-by: Maximilian Meister mmeister@suse.de (cherry picked from commit 34c7b11c6cfacd955701c7fefae17d7b22d6b7eb)

Backport of https://github.com/kubic-project/velum/pull/608