cli: maas logout causes KeyError for other profiles

maas cli commands fail if run at the same time as 'maas logout' for any other profile.

The setup where this bug is frequently seen involves a host which interacts with 4 separate MAAS servers. The host is a test driver and uses the maas cli, when a test is being run, to collect information about the nodes that have been allocated:
    1. maas login maas_A http://10.0.40.100/MAAS/api/1.0/ <apikey>
    2. <maas cli commands to query nodes>
    3. maas logout maas_A

Tests are run in parallel against all 4 MAAS server (maas_A, maas_B, maas_C, maas_D).

When the timing lines up is a way that 'maas logout' is called, for example against maas_A, within close proximity to maas cli commands against maas_B, maas_C or maas_D, the following error is reported back to the shell running the latter.

jenkins@finfolk:~$ maas maas_B nodes list
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/usr/lib/python2.7/dist-packages/maascli/__main__.py", line 20, in <module>
    main()
  File "/usr/lib/python2.7/dist-packages/maascli/__init__.py", line 41, in main
    parser = prepare_parser(argv)
  File "/usr/lib/python2.7/dist-packages/maascli/parser.py", line 80, in prepare_parser
    api.register_api_commands(parser)
  File "/usr/lib/python2.7/dist-packages/maascli/api.py", line 480, in register_api_commands
    profile = config[profile_name]
  File "/usr/lib/python2.7/dist-packages/maascli/config.py", line 56, in __getitem__
    raise KeyError(name)
KeyError: u'maas_foo'

This bug can easily be re-produced by:
    1. Open two shells on the same host and log into two different MAAS servers.
    2. From one shell call 'maas logout <profile>'
    3. Immediately attempt to run any maas cli command from the second shell.

A re-try will generally work, so it seems there is a small window when this occurs but not so small that it's hard to reproduce.

ii maas-cli 1.9.0~alpha4+bzr4371-0ubuntu1~trusty1 all MAAS command line API tool
un maas-region-controller-min <none> <none> (no description available)
ii python-maas-client 1.9.0~alpha4+bzr4371-0ubuntu1~trusty1 all MAAS python API client