More location customisation aids

* Sets the caller context on same server entities to the entity rather
  than the lifecycle tasks class
* Invokes JcloudsLocationCustomizer instances with the ConfigBag being
  used to obtain machines if such a constructor exists, otherwise
  invokes the class' no-argument constructor.
Merge pull request #1308 from sjcorbett/location-customisation

More JcloudsLocation customisation aids