Remove swiftkerbauth.py from tox.ini

Change-Id: Ib73e0f86a8c6d80495ab966255f9143b20cdfb90
Signed-off-by: Prashanth Pai <ppai@redhat.com>
Reviewed-on: http://review.gluster.org/6038
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>