libprocessgroup: convert Json::Value to const refs

Convert Json::Value to const refs to prevent creation of a null member
if specified element does not exist. Fix variable naming in the parser
while we are at it.

Change-Id: I7dabfd1c73983b465984dd6c411c9fe48743a348
Signed-off-by: Suren Baghdasaryan <surenb@google.com>