Add "server group create" command

Support compute v2 "server group create" command in OSC.

Implements: blueprint nova-server-group-support
Partial-Bug: #1542171

Change-Id: I96ffb07764d3adb715e048943cfee3b879c280f6

doc/source/command-objects/server-group.rst

@@ -0,0 +1,29 @@
+============
+server group
+============
+
+Server group provide a mechanism to group servers according to certain policy.
+
+Compute v2
+
+server group create
+-------------------
+
+Create a new server group
+
+.. program:: server group create
+.. code-block:: bash
+
+    os server group create
+        --policy <policy> [--policy <policy>] ...
+        <name>
+
+.. option:: --policy <policy>
+
+    Add a policy to :ref:`\<name\> <server_group_create-name>`
+    (repeat option to add multiple policies)
+
+.. _server_group_create-name:
+.. describe:: <name>
+
+    New server group name

doc/source/commands.rst

@@ -118,6 +118,7 @@ referring to both Compute and Volume quotas.
 * ``security group rule``: (**Compute**, **Network**) - the individual rules that define protocol/IP/port access
 * ``server``: (**Compute**) virtual machine instance
 * ``server dump``: (**Compute**) a dump file of a server created by features like kdump
+* ``server group``: (**Compute**) a grouping of servers
 * ``server image``: (**Compute**) saved server disk image
 * ``service``: (**Identity**) a cloud service
 * ``service provider``: (**Identity**) a resource that consumes assertions from an ``identity provider``