VMware: Allow user to specify soft or hard reboot

Nova api allows user to specify soft or hard reboot an instance. However, VMware driver behavior is to attempt soft reboot first, if it can not do soft reboot, then it will do hard reboot. And current VMware driver code does not execute hard reboot directly, if user specify hard reboot in api request.
Since hard reboot may result in disk data inconsistency, user who is running critical application in a cloud instance may refuse hard reboot. Need to make VMware driver reboot code allows user to specify soft/hard, and ensure hard reboot will not be executed after soft reboot fails.