Rename server arg in wait_for_interface_status method

This is very minor, but the server formal argument to the
wait_for_interface_status method is actually a server id.

This changes the name of the parameter to be clear on the
type of that object, since it could easily be confused
with a full server response body dict.

Change-Id: Id7cab21ab75092226f0784877ae55f17e880229c
diff --git a/tempest/common/waiters.py b/tempest/common/waiters.py
index 3e5600c..9c83c99 100644
--- a/tempest/common/waiters.py
+++ b/tempest/common/waiters.py
@@ -261,16 +261,16 @@
         time.sleep(client.build_interval)
 
 
-def wait_for_interface_status(client, server, port_id, status):
+def wait_for_interface_status(client, server_id, port_id, status):
     """Waits for an interface to reach a given status."""
-    body = (client.show_interface(server, port_id)
+    body = (client.show_interface(server_id, port_id)
             ['interfaceAttachment'])
     interface_status = body['port_state']
     start = int(time.time())
 
     while(interface_status != status):
         time.sleep(client.build_interval)
-        body = (client.show_interface(server, port_id)
+        body = (client.show_interface(server_id, port_id)
                 ['interfaceAttachment'])
         interface_status = body['port_state']