The static zuul driver returns only a public_ipv4 address for
nodes in use. The multinode bridge role accesses the node
private_ipv4 address. As such, when the private address is
empty/null the role fails.
This review defaults that private address so that the role
uses the public address when the private address is null.
Change-Id: I32fb17bae98f13f735da4d5b9a6a01e948f21678