dockertest.images: Add default image FQIN property

Simply a convenience, since the majority of the time
callers use ``DockerImage.full_name_from_defaults(self.config)`` and
this class already has access to the subtest configuration.

Signed-off-by: Chris Evich <cevich@redhat.com>
Docker: Add negative usage test

Signed-off-by: Chris Evich <cevich@redhat.com>
subtests: Fix minor exception message typo

Signed-off-by: Chris Evich <cevich@redhat.com>