CLI argument validation utility

In nova-manage and cinder-manage, we allow command arguments to be passed as optional or positional arguments.

e.g.

  $> nova-manage floating create 10.0.0.1/28
  $> nova-manage floating create --ip_range 10.0.0.1/28

are equivalent.

Once nova-manage has collected those arguments, it calls the appropriate command function with them as positional and keyword arguments. If the user forgets to supply a required argument, they merely get a TypeError with little useful information.

We should improve the usability of these commands with a utility function to check that the required arguments have been supplied and raise a useful exception if not.