Correct login name deduction in SshServer

Currently SshServer ("server ssh" command) raises an AttributeError if
no explicit login is provided because it uses an attribute which no more
exists.

Change-Id: I86f68230037d51efb41aca62e07e058733ecd67a
Closes-Bug: #1624085

Author: ZZelle

openstackclient/compute/v2/server.py

@@ -1677,7 +1677,7 @@ def take_action(self, parsed_args):
         if parsed_args.login:
             login = parsed_args.login
         else:
-            login = self.app.client_manager._username
+            login = self.app.client_manager.auth_ref.username
         if parsed_args.verbose:
             cmd += " -v"