"nova list" will also output "Task State" and "Power State" by default.
This patch improves "server list" command to have the same columns, but
not by default. These two columns will be output if --long is added.
The power state is an int, so also adds a formatter helper function
to translate it to human readable string, just as "Networks" does.
Change-Id: I0530a910bec03835839a5ba7687c66d5643338f3