[#167] netmap: Allow to select insufficient number of nodes

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

netmap/context.go

@@ -40,6 +40,10 @@ type context struct {
 	// policy uses the UNIQUE flag. Nodes marked as used are not used in subsequent
 	// base selections.
 	usedNodes map[uint64]bool
+
+	// If true, returns an error when netmap does not contain enough nodes for selection.
+	// By default best effort is taken.
+	strict bool
 }
 
 // Various validation errors.