Cosmetic fix - replace hard-coded fast-reboot with REBOOT_TYPE variable

[vaibhavhd]

- What I did
Fixes sonic-net/sonic-buildimage#5980

One of the logging statements during warm-reboot incorrectly prints fast-reboot.

- How I did it
Remove hard-coded fast-reboot with REBOOT_TYPE variable

- How to verify it
Modified the script in device, and tested that logging is corrected:

- Previous command output (if the output of a command-line utility has changed)

admin@sonic-device:~$ sudo warm-reboot -vvv
Thu 19 Nov 2020 08:23:46 PM UTC Pausing orchagent ...
Thu 19 Nov 2020 08:23:47 PM UTC Collecting logs to check ssd health before fast-reboot...
...
...
Thu 19 Nov 2020 08:24:13 PM UTC Enabling Watchdog before warm-reboot

- New command output (if the output of a command-line utility has changed)

admin@sonic-device:~$ sudo warm-reboot -vvv
Thu 19 Nov 2020 10:39:04 PM UTC Pausing orchagent ...
Thu 19 Nov 2020 10:39:05 PM UTC Collecting logs to check ssd health before warm-reboot...
...
...
Thu 19 Nov 2020 10:39:25 PM UTC Enabling Watchdog before warm-reboot