Since the upstream graylogctl script will prefer finding its java
executable based on JAVA_HOME, we now set this instead of PATH in
order to allow it to find the JRE. By setting it conditionally on it
not already being set, we allow selecting a different JRE at runtime.
We also explicitly use openjdk11, which supports the
UseConcMarkSweepGC option which graylog insists on using.
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