MB-100: perform_verify_queries on self.master

in some cases servers[0] is not master anymore, for example in online_upgrade_rebalance_in_out

Change-Id: I8f119ed17717bd3aee4f6de38fe82176e5b2ee33
Reviewed-on: http://review.couchbase.org/24129
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>