Cloud providers may only return "public" IPs

For example, Bluelock vCloud only has one IP per instance, which is returned as the public address. In these sorts of situations, where only a "public" address exists in the NodeMetadata, we should fall back to that public address rather than giving up because there's no private address.

2012-08-01 20:15:08,029 ERROR [org.apache.whirr.ClusterController] (main) Unable to start the cluster. Terminating all nodes.
java.lang.IndexOutOfBoundsException: index (0) must be less than size (0)
at com.google.common.base.Preconditions.checkElementIndex(Preconditions.java:305)
at com.google.common.base.Preconditions.checkElementIndex(Preconditions.java:284)
at com.google.common.collect.Iterables.get(Iterables.java:734)
at org.apache.whirr.actions.BootstrapClusterAction$1.apply(BootstrapClusterAction.java:142)