Flume - allow -D and -X expressions to be inserted as part of the flume global config

[Alex-Ikanow]

So can fix the memory (eg want -Xmx100m) from the global config without having to hand code inside the flume-ng binary as I had to at present (which then gets wiped...)

(Alternatively set JAVA_OPTs or FLUME_JAVA_OPTs)