Add additional set_nodes/get_nodes functions

These functions will allow to configure a provider with a specific list
of nodes, and retrieve the current node list.

Change-Id: Iba94fe73a8b4328039f1fcdf8fcd31ad20019fd8
Reviewed-on: http://review.couchbase.org/37338
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>