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
Added GROUP_MEMBERS attribute to AbstractGroup
More helper methods to create ConfigKeys
Fix JAVA_OPTS replacement in environment for ActiveMQ and Jetty
Typo in messages for JMXRMI agent
Use JAVA_OPTS when launching ZooKeeper
Tidying up the ZooKeeper driver code

- Formatting and imports
- Use existing attributeSupplierWhenReady
Remove duplicate line when wrapping entity spec
Fix synchromization in CountdownTimerTest
Tidying up SSH driver new script with USE_PID_FILE constant
Tidying Cassandra driver script invocation command
Use supplier to resolve address of host
Short-circuit test for isJmxSslEnabled
Update CassandraNode driver with latest Brooklyn methods and revert change to scriptInvocationCommand
Add test assertions for GROUP_MEMBERS attribute
Add prepend StringFunction mirroring append
Further SSH driver tidying and canonicalisation across entities
Updating the Storm entity
Add classpath to java app launch