Expand help message for 'migrate' to explain how the new host is sele…

…cted

Fixes bug 1078247

Change-Id: Iedfd6fc957fd1c2e53f7685b02bd33e16d9342e9

novaclient/v1_1/shell.py

@@ -810,7 +810,7 @@ def do_resize_revert(cs, args):
     default=False,
     help='Blocks while instance migrates so progress can be reported.')
 def do_migrate(cs, args):
-    """Migrate a server."""
+    """Migrate a server. The new host will be selected by the scheduler."""
     server = _find_server(cs, args.server)
     server.migrate()