conn.py: allow any params in Test Conn's _cmd

The _cmd() methods got additional arguments that could not be parsed by the
test connection classes. Therefore the test connection classes got
*args,**kwargs added, so that they can also handle additional parameters that
might be added in the future.

See GitHub Issue #80.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
Acked-by: Eik Binschek <binschek@dresearch-fe.de>
Merge branch 'b-cmd-returncode' into dev

Resolves GitHub Issue #80.