Adds CloudExplorer utility

Convenience for listing Cloud Compute and BlobStore details.

For fuller functionality, consider instead the jclouds CLI or Ruby Fog CLI.

The advantage of this utility is that it piggie-backs off the {@code brooklyn.property} credentials, so requires less additional credential configuration. It also gives brooklyn-specific information, such as which image will be used by default in a given cloud.

Also some minor tidies to brooklyn Main.