presence: Use new JsonConfig object

To simplify handling the loading of config files, use the updated
JsonConfig object that populates the available compatibility values used
when retrieving the JSON file and loading it. The given load function is
called if compatibility values are found upon being constructed or after
an interfacesAdded signal is received, which then it can call
`getConfFile` to find the JSON config file to be loaded.

Change-Id: I7698f0abf91dc6414386a42b710bcde9c3f47612
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
diff --git a/presence/json_parser.hpp b/presence/json_parser.hpp
index 857a2f8..fc60229 100644
--- a/presence/json_parser.hpp
+++ b/presence/json_parser.hpp
@@ -78,10 +78,8 @@
      * @brief Parses and populates the fan presence policies from
      *        the json file and then starts the actual presence
      *        detecting.
-     *
-     * @param[in] confFile - The conf file name to use
      */
-    void start(const std::string& confFile);
+    void start();
 
   private:
     /* Fan presence policies */