EXTERNAL_MODULES is not all that descriptive. It should relate to what the modules being loaded do.

The default value should also be blank so that it doesn't start anything unless explicitly configured to do so (which obviates the need for the `ENABLE_EXTERNAL_MODULES` configuration option. It also will be confusing to users when they set that configuration field with their own module, and kafka no longer loads. Lastly blueflood-core should not depend upon any other blueflood module, and having a default value here that requires blueflood-kafka jar to be present will cause blueflood to fail in unexpected ways.