Add britney option: --series

- autopkgtest tests rely on it
- Ubuntu uses this

britney.py

@@ -413,6 +413,8 @@ class Britney(object):
                           help="Compute which packages can migrate (the default)")
         parser.add_option("", "--no-compute-migrations", action="store_false", dest="compute_migrations",
                           help="Do not compute which packages can migrate.")
+        parser.add_option("", "--series", action="store", dest="series", default='series',
+                               help="set distribution series name")
         (self.options, self.args) = parser.parse_args()
         
         # integrity checks