Allow ANONYMOUS sasl_mechanism by default

Kombu currently only uses "PLAIN" authentication by default. However, Qpid does
not set up a SASL database on a default install. This means users have to
either set up their own SASL db, or set `auth=no` in qpidd.conf.

For a better out-of-the-box experience, both PLAIN and ANONYMOUS mechanisms
should be supported by kombu. See
https://issues.apache.org/jira/browse/QPID-6091 from @bmbouter  for more detail.

We believe that this does not impact security since the default
login is already set to "guest/guest". Allowing anonymous auth is nearly the
same from a security standpoint.