Multiple API version support

* Use multiple entry point groups to represent each API+version
  combination supported
* Add some tests

Try it out:
* Right now only '* user' commands have multiple overlapping versions;
  you can see the selection between v2.0 and v3 by looking at the
  command help output for 'tenant' vs 'project':

  os --os-identity-api-version=2.0 help set user
  os --os-identity-api-version=3 help set user

Change-Id: I7114fd246843df0243d354a7cce697810bb7de62

doc/source/commands.rst

@@ -0,0 +1,60 @@
+========
+Commands
+========
+
+Command Structure
+=================
+
+OpenStack Client uses a command form ``verb object``.
+
+Note that 'object' here refers to the target of a command's action.  In coding
+discussions 'object' has its usual Python meaning.  Go figure.
+
+Commands take the form::
+
+    openstack [<global-options>] <verb> <object> [<command-local-arguments>]
+
+Command Arguments
+-----------------
+
+  * All long option names use two dashes ('--') as the prefix and a single dash
+    ('-') as the interpolation character.  Some common options also have the
+    traditional single letter name prefixed by a single dash ('-').
+  * Global options generally have a corresponding environment variable that
+    may also be used to set the value. If both are present, the command-line
+    option takes priority. The environment variable names can be derived from
+    the option name by dropping the leading '--', converting all embedded dashes
+    ('-') to underscores ('_'), and converting the name to upper case.
+  * Positional arguments trail command options. In commands that require two or
+    more objects be acted upon, such as 'attach A to B', both objects appear
+    as positional arguments. If they also appear in the command object they are
+    in the same order.
+
+
+Implementation
+==============
+
+The command structure is designed to support seamless addition of extension
+command modules via entry points.  The extensions are assumed to be subclasses
+of Cliff's command.Command object.
+
+Command Entry Points
+--------------------
+
+Commands are added to the client using distribute's entry points in ``setup.py``.
+There is a single common group ``openstack.cli`` for commands that are not versioned,
+and a group for each combination of OpenStack API and version that is
+supported.  For example, to support Identity API v3 there is a group called
+``openstack.identity.v3`` that contains the individual commands.  The command
+entry points have the form::
+
+    "verb_object=fully.qualified.module.vXX.object:VerbObject"
+
+For example, the 'list user' command fir the Identity API is identified in
+``setup.py`` with::
+
+    'openstack.identity.v3': [
+        # ...
+        'list_user=openstackclient.identity.v3.user:ListUser',
+        # ...
+    ],