Add some params docstring to create_server

Basically we don't describe all available params on the docstring to
avoid maintenance burden, but we add some params to the method which
calls "create a server" API because the API is most useful in whole
OpenStack APIs, and the added params are mandatory in most use cases.

Change-Id: If15080b5950c98207c10de2839c724fda07ff91f
diff --git a/tempest/lib/services/compute/servers_client.py b/tempest/lib/services/compute/servers_client.py
index 8b22be0..d5902e1 100755
--- a/tempest/lib/services/compute/servers_client.py
+++ b/tempest/lib/services/compute/servers_client.py
@@ -47,8 +47,13 @@
     def create_server(self, **kwargs):
         """Create server.
 
-        Available params: see http://developer.openstack.org/
-                              api-ref-compute-v2.1.html#createServer
+        For a full list of available parameters, please refer to the official
+        API reference:
+        http://developer.openstack.org/api-ref/compute/#create-server
+
+        :param name: Server name
+        :param imageRef: Image reference (UUID)
+        :param flavorRef: Flavor reference (UUID or full URL)
 
         Most parameters except the following are passed to the API without
         any changes.