RDC Prometheus plugin return errors when use the --rdc_gpu_indexes

When above option is used, the plugin returns errors:
  result = rdc.rdc_group_gpu_add(rdc_handle, gpu_group_id, gpu)
  ctypes.ArgumentError: argument 3: <type 'exceptions.TypeError'>: wrong type

The rdc_prometheus.py is changed to convert string to integer.
The RdcUtil.py is also changed to raise Exception properly.

Change-Id: I9535091ff1fc8882cccd32e5f2810da5241768c3

python_binding/RdcUtil.py

@@ -54,13 +54,13 @@ class RdcUtil:
         gpu_group_id = c_uint32()
         result = rdc.rdc_group_gpu_create(rdc_handle, rdc_group_type_t.RDC_GROUP_EMPTY, gpu_group_name, gpu_group_id)
         if rdc_status_t(result) != rdc_status_t.RDC_ST_OK:
-            raise Exception("Fail to create the GPU group " << group_name)
+            raise Exception("Fail to create the GPU group " + group_name)
 
         #Add GPU index to the group
         for gpu in gpu_indexes:
             result = rdc.rdc_group_gpu_add(rdc_handle, gpu_group_id, gpu)
             if rdc_status_t(result) != rdc_status_t.RDC_ST_OK:
-                raise Exception("Fail to add GPU index " << gpu <<" to group " << gpu_group_id)
+                raise Exception("Fail to add GPU index " + str(gpu) + " to group " + str(gpu_group_id))
 
         return gpu_group_id, True
 
@@ -75,7 +75,7 @@ class RdcUtil:
             group_info = rdc_field_group_info_t()
             result = rdc.rdc_group_field_get_info(rdc_handle, field_group_id_list[index], pointer(group_info))
             if rdc_status_t(result) != rdc_status_t.RDC_ST_OK:
-                raise Exception("Fail to get field group " << field_group_id_list[index] <<" info")
+                raise Exception("Fail to get field group " + str(field_group_id_list[index]) + " info")
             if group_info.group_name.decode("utf-8") == field_group_name:
                 field_ids_ori = [ e.value for  e in group_info.field_ids[:group_info.count] ]
                 # reuse the old field group
@@ -84,7 +84,7 @@ class RdcUtil:
                 else:
                     result = rdc.rdc_group_field_destroy(rdc_handle, field_group_id_list[index])
                     if rdc_status_t(result) != rdc_status_t.RDC_ST_OK:
-                        raise Exception("Fail to delete field group " << field_group_id_list[index])
+                        raise Exception("Fail to delete field group " + str(field_group_id_list[index]))
 
         #Create new field group
         fields_c_ids = []