incorrect way to detect fast-reboot

[rameshsanth]

inconsistent strings used in /proc/cmdline to detect fast reboot.
dockers/docker-orchagent/swssconfig.sh uses "fast-reboot" to detect fast reboot

Looks like SONIC_BOOT_TYPE=fast is the string that is saved by sonic-utilities:scripts/fast-reboot file

Correct usage
docker_image_ctl.j2 checks for "SONIC_BOOT_TYPE=fast" in cmdline

[qiluo-msft]

The doc https://github.com/Azure/SONiC/blob/master/doc/warm-reboot/system-warmboot.md#details-of-kernel-arguments explain the design choice we made. Because of backward-compatibility issue, we made it inconsistent on purpose.

Let me know if you have more questions on the design doc.