bp: Always require local_alert key argument to have a node.

Change-Id: I7238ec6500c1f0c462762779bf575b198fb3c030
Orig-Reviewed-on: http://review.couchbase.org/37206
Orig-Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Orig-Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-on: http://review.couchbase.org/42853
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>