negotiate: handle cases where no mechanisms are found

This happens in raw memcached daemons where there is no authentication
configured for the instance.

Change-Id: I18085ba3164d3465b35dbce98c19be5695cb38d2
Reviewed-on: http://review.couchbase.org/38750
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
vbucket: allow to convert a config to ketama

This is used in conjunction with lcbvb_genconfig_ex to create a new
memcached bucket config

Change-Id: I76c973a94d346a70300fd9df20dd16068c5d8c32
Reviewed-on: http://review.couchbase.org/38751
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>