chassishandler: Add restart cause support

Added support for chassis restart cause command. This will read data
from Host power state dbus interface and reply to ipmi command.

Tested: Verified executing 'ipmitool chassis restart_cause'
System restart cause: power-up via pushbutton

Signed-off-by: Vijay Khemka <vijaykhemka@fb.com>
Change-Id: I8e351035da1b8b1d85f3c34094fd7533e7af529b
1 file changed