Fixes#3096
The credentials processing for transitional auth was broken
in ba6a41d, "auth: Switch to sharded service which effectively removed
the "virtualization" of underlying auth in the SASL challenge.
As a quick workaround, add the permissive exception handling to
sasl object as well.
Message-Id: <20180103102724.1083-1-calle@scylladb.com>