Call for tokens in "latest" like the other sub-commands

[rafaelcaricio]

We should be uniform in how to get tokens to use in subcommands. User reported:

Traceback (most recent call last):
  File "/usr/local/bin/pierone", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.5/site-packages/pierone/cli.py", line 342, in main
    cli()
  File "/usr/local/lib/python3.5/site-packages/click/core.py", line 716, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.5/site-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.5/site-packages/click/core.py", line 1060, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.5/site-packages/click/core.py", line 889, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.5/site-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.5/site-packages/click/decorators.py", line 27, in new_func
    return f(get_current_context().obj, *args, **kwargs)
  File "/usr/local/lib/python3.5/site-packages/pierone/cli.py", line 214, in latest
    print(get_latest_tag('pierone', image))
  File "/usr/local/lib/python3.5/site-packages/pierone/api.py", line 105, in get_latest_tag
    raise Unauthorized()
pierone.api.Unauthorized: Unauthorized: token missing or invalid

But other sub-commands work fine, like "pierone tags". The only reason it is not working for the latest sub-command is because it calls for the pierone token in a different way than other sub-commands. This PR makes latest call for tokens the same way.

[coveralls]

Coverage decreased (-0.4%) to 81.044% when pulling 6a04129 on rafaelcaricio:use-same-code-for-latest into 991c05e on zalando-stups:master.

[hjacobs]

👍

[whiskeysierra]

Temporary workaround (requires jq):

pierone tags stups zmon-aws-agent -o json | jq -r '.[length - 1].tag'