All *SkipTests are now inherited from "SkipTest" baseclass. This
baseclass contains improved __init__ which removes any unsupported
parameters from the Test.__init__ super call.

This is because those classes are used to substitute other avocado test
classes, for example "avocado-vt" tests which contain "vt_params"
argument unsupported by Test.__init__.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>