Explicitly specify region in Rackspace GenerateTempURL example

This PR updates the `GenerateTempURL` example to specifically use the `RegionScopedBlobStoreContext` APIs that take a region argument, i.e. `RegionScopedBlobStoreContext.getBlobStore()` -> `RegionScopedBlobStoreContext.getBlobStore(REGION)`

Imports have been cleaned up, and the creation of a directory has been added to ensure the example runs standalone.

JIRA: https://issues.apache.org/jira/browse/JCLOUDS-592