Image id is not populated in nodes that have not been created by jclouds

The InstancetoNodeMetadata uses a "boot disk -> image" cache that is populated by the compute service adapter when nodes are created.

When there are existing nodes in the account that were already there, those nodes don't get the image id field populated. In the cases where the cache does not contain a node's boot disk, jclouds should perform the necessary API callls to get the image id.