1099954 - Adds cyrus-sasl-plain note for SASL config to user guide

https://bugzilla.redhat.com/show_bug.cgi?id=1099954

It took me a while to test this, but I've determine that cyrus-sasl-plain only needs to be installed on the broker and NOT on the pulp server. As such there was only one place to update it.

Personally I feel this is redundant with the Qpid docs, but we spend time with users helping them resolve these issues so I've documented the process from a high level here anyway.