Enhance the variable replacement

Add ability to also run shell commands contained in the sys.config.
E.g.:

# In this example the node name defaults to name of the release
# and can be overriden at run-time, appended with current year
$ head -1 vm.args
-sname ${NODE_NAME:-$REL_NAME}$(date +%Y)

# If the $NAME is 'abc2015', and hostname is 'MyHost', below the 'node'
# parameter gets set to 'Abc2015@myhost'
$ grep node sys.config
   {node, $(echo ${NAME^})@${HOSTNAME,,}}

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