Add support for configuring variables at a Process Group level. Variables, like Controller Services, are scoped by the group they are defined in and are available to any Processor defined there and below (ie and descendant Processors). Variables in a descendant group can override the value in a parent group. Variable permissions will be based solely on the corresponding Process Group.
Because variables are specified in EL, there is no controlling access to them like Controller Services which can be restricted using the combo selector. Because of this, variables should not be used for sensitive values.