Support multiple argument in identity command-line

In OSC, most commands can only operate on one object at one time. For example, you cannot create two servers within one single command.

There was a BP going to support operation on multiple servers within one command.
( https://blueprints.launchpad.net/python-openstackclient/+spec/cmd-with-multi-servers )
But the work stopped because of the discussion of error handling process in multiple rest api calls.

Now, in doc/source/command-errors.rst, we have defined the behavior of error handling in multiple rest api calls situation. So, we can restart the work of supporting multiple operation objects in command-line.

Instead of using the old BP, I'd like to propose a new one here to support not only server commands, but also all the commands possible in OSC.

This BP is for tracing identity commands.
( Will update the detailed work to do here. )

Related BP:
https://blueprints.launchpad.net/python-openstackclient/+spec/multi-argument-compute
https://blueprints.launchpad.net/python-openstackclient/+spec/multi-argument-network