AttributeError: 'Namespace' object has no attribute 'only_ids'

[Zygmunt Krynicki]

The "plainbox session" command crashes ever since we added the --only-ids option

(venv)zyga@g580:~/checkbox.here/plainbox/plainbox/impl/providers/stubbox$ plainbox session
CRITICAL plainbox.crashes: Plik wykonywalny 'plainbox' uruchomiony z Namespace(command=<plainbox.impl.commands.cmd_session.SessionCommand object at 0x7f4bea06c710>, debug_console=False, debug_interrupt=False, default_session_cmd='list', log_level=None, pdb=False, trace=[]) uległ awarii
Traceback (most recent call last):
  File "/home/zyga/checkbox.here/plainbox/plainbox/impl/clitools.py", line 548, in dispatch_and_catch_exceptions
    return self.dispatch_command(ns)
  File "/home/zyga/checkbox.here/plainbox/plainbox/impl/clitools.py", line 544, in dispatch_command
    return ns.command.invoked(ns)
  File "/home/zyga/checkbox.here/plainbox/plainbox/impl/commands/cmd_session.py", line 59, in invoked
    return SessionInvocation(ns, self.provider_loader).run()
  File "/home/zyga/checkbox.here/plainbox/plainbox/impl/commands/inv_session.py", line 58, in run
    self.list_sessions()
  File "/home/zyga/checkbox.here/plainbox/plainbox/impl/commands/inv_session.py", line 72, in list_sessions
    if self.ns.only_ids:
AttributeError: 'Namespace' object has no attribute 'only_ids'

This is caused by the fact that argparse doesn't understand what we're doing and doesn't give us any defaults for sub-parsers we're not running.