[testrunner_api] Function to find out what node is the master.
[testrunner_api] Function to find out what node is the master.

Change-Id: I42e17ec474996f55c83e44b31525461c0652c1d3
Reviewed-on: http://review.couchbase.org/89059
Tested-by: Aliaksey Artamonau <ce14b1c16e0fd654f926118eb8addbec5aff4ffa@couchbase.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server