JCLOUDS-277: Made the description in org.jclouds.compute.domain.Image nullable
Skip unimplemented test with transient blobstore
JCLOUDS-269: Report ram as MB rather than GB for SoftLayer
JCLOUDS-282: Add NL to assignable locations for SoftLayer

- zone contained NL, so needed to add it to SoftLayerProviderMetadata's
  iso3166Codes
JCLOUDS-267. Add SecurityGroupExtension support for Nova.