use the usr/pwd from mcs_server_st, if available, for SASL auth
use the usr/pwd from mcs_server_st, if available, for SASL auth

The user/password auth credential information can now pass from the
JSON (re-)configuration message, to libvbucket, to mcs_server_st, and
finally (with this change) to the cproxy_auth_downstream() function.

Change-Id: I6be68fcb7a33422aaa92b646cf7a4516a8702626
Reviewed-on: http://review.northscale.com/1375
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

zbase/moxi,membase/moxi,couchbase/moxi,membase/moxi,steveyen/moxi,couchbase/moxi,steveyen/moxi,membase/moxi,zbase/moxi,membase/moxi,membase/moxi,couchbase/moxi,zbase/moxi,couchbase/moxi,couchbase/moxi,steveyen/moxi,couchbase/moxi,zbase/moxi,zbase/moxi,steveyen/moxi,membase/moxi,steveyen/moxi