init: Add support for reboot_delay cmdline args

This patch adds support for configuring reboot_delay in cmdline args:
reboot_delay=number
to set specify the amount of secnods to wait before rebooting.
This informations are used to set reboot_delay=number during
an auto-installation.

Change-Id: Iee9888ddfd0323e37a6c8524043cf4e21e9e2166
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1021437
Signed-off-by: hadong <hadong@redhat.com>
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>