Misc improvements and Tidyup

Miscellaneous field improvements rollup, including:

- New `GROUP_MEMBERS` attribute (for @alasdairhodge as a precursor to rendering entities as clickable links in the UI, this shows a list of group member IDs for things like `DynamicMultiGroup`)
- Some more helpers to create the `BasicAttributeSensorAndConfigKey` just like other `ConfigKey`s
- Tidying up the use of *JAVA_OPTS* replacements in some of the Java based entities
- Adds *JAVA_OPTS* support to `ZooKeeperNode` (so that Waratek can start it in a container)
- Fixes a deadlock in `CountdownTimerTest` that was hanging builds on CloudBees
- More tidying SSH drivers and adding `USE_PID_FILE` constant

View the [Files Changed](https://github.com/brooklyncentral/brooklyn/pull/1299/files?w=1) with the `?w=1` option to hide whitespace-only changes from the diffs.