Simplify Redfish error message property interface

The error message code used a json_pointer object which must
begin with a '/' character and had to be sent as an extra
parameter.  This change simplifies the interface by using a
string so there doesn't have to be a '/'.  This allowed the
same property argument passed for the message to be used
for the property field path.

Tested: Sent an error with a property and verified that it is
correctly displayed.

Change-Id: I0571e2eee627cedf29d751588a4f1bceee66f084
Signed-off-by: Jason M. Bills <jason.m.bills@linux.intel.com>
diff --git a/redfish-core/lib/redfish_sessions.hpp b/redfish-core/lib/redfish_sessions.hpp
index b0a2948..68449c0 100644
--- a/redfish-core/lib/redfish_sessions.hpp
+++ b/redfish-core/lib/redfish_sessions.hpp
@@ -171,12 +171,12 @@
         {
             if (username.empty())
             {
-                messages::propertyMissing(res, "UserName", "UserName");
+                messages::propertyMissing(res, "UserName");
             }
 
             if (password.empty())
             {
-                messages::propertyMissing(res, "Password", "Password");
+                messages::propertyMissing(res, "Password");
             }
             res.end();