[BUGFIX] CommandController must reset its arguments on dispatch (Revision 1)

The CLI CommandController is not stateless in the sense that it
just adds argument definitions without removing previous definitions
in ``processRequest()``.
Usual this is not a problem, because every command is called only
once per request. But in tests or when using the ``flow:core:shell``
command this has weird side effects.

Change-Id: If65c5cf8ac394a96211eb48652ff155af44a6662
Releases: master, 2.2, 2.1
Revision-Id: 51468faca43f66224723e1ffe788b9bcb67d7cf8