Add a 'data-exec' directive support to the JSON PMDA.
Add a 'data-exec' directive support to the JSON PMDA.

* src/pmdas/json/pmdajson.python (__load_json_data): Add the capability to
  run a command to get the JSON data.
  (load): Call new __preparse_metadata function to parse metadata looking
  for items needed to find the JSON data.
  (__preparse_metadata): New function.

prasincs/pcp,prasincs/pcp,tjanez/pcp,edwardt/pcp,aeg-aeg/pcpfans,tjanez/pcp,aeg-aeg/pcpfans,edwardt/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,wuliming/pcp,prasincs/pcp,wuliming/pcp,edwardt/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,aeg-aeg/pcpfans,edwardt/pcp,edwardt/pcp,prasincs/pcp,tjanez/pcp,tjanez/pcp,tjanez/pcp,wuliming/pcp,aeg-aeg/pcpfans,adfernandes/pcp,edwardt/pcp,tjanez/pcp,prasincs/pcp,edwardt/pcp,wuliming/pcp,adfernandes/pcp,edwardt/pcp,wuliming/pcp,adfernandes/pcp,prasincs/pcp,prasincs/pcp,wuliming/pcp,aeg-aeg/pcpfans,prasincs/pcp,tjanez/pcp,tjanez/pcp