CBQE-1101: attach postcondition handler at prerun

Change allows specifying which condition checking
method should be picked up by the postcondition_handler
when a workload is started

Change-Id: I54f2a2b956bd8f006e80405efb042d878a130a78
Reviewed-on: http://review.couchbase.org/25175
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>