Utility function to set the master node in a node list

The information about which node is the master node is a cluster-wide
setting, in most formats provided independently of the node information.
Most backends therefore have to set the isMaster bit indepently in the
obtained node list. This patch provides a utility function for this.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>