rename "dropin" dir to "dropins"
Merge pull request #1300 from aledsage/fix/rename-dropin-to-dropins

rename "dropin" dir to "dropins"
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