warn against sorting requirements

And do not check for sorted requirements.

Change-Id: I64ae9191863564e278a35d42ec9cd743a233028e
Closes-Bug: 1365061
Add --show-nested option to stack-list

Adds an option to display nested stacks, provided the heat service
being called supports the show_nested argument.  In this mode we
display the parent stack ID as an additional column, a future TODO
is to add another switch which enables a tree/forest view as well,
which should be more friendly to human readers of the output.

Change-Id: Ic559d6c6467b1a2bc1058d792b99fea483cdfb65
blueprint: list-nested
Merge "Add --show-nested option to stack-list"
Merge "warn against sorting requirements"