CCBC-243 Substitute isasl with cbsasl

cbsasl has implemented both PLAIN and CRAM-MD5 authentication
mechanisms

Change-Id: I7fcb19a59951a65f363bc66bd0375c67c2a0d468
Reviewed-on: http://review.couchbase.org/29627
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>