MB-10400: added ns_server_testrunner_api:shutdown_nicely/0
MB-10400: added ns_server_testrunner_api:shutdown_nicely/0

I.e. for testing clean shutdown of couchbase

Change-Id: I6830af486880a823be8ac6b958a9e03bb7b53645
Reviewed-on: http://review.couchbase.org/34807
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

nimishzynga/ns_server,mschoch/ns_server,membase/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,daverigby/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,mschoch/ns_server,daverigby/ns_server,ceejatec/ns_server,nimishzynga/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,membase/ns_server,membase/ns_server,t3rm1n4l/ns_server,membase/ns_server,nimishzynga/ns_server,membase/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,nimishzynga/ns_server,daverigby/ns_server