Filter out the path variable of field

One RSD resource may contains several fields that also have _path
variable. That is useless info to user. So filter them out.

Change-Id: Idf453f452603713c8efd64336eee791c656a1d86

requirements.txt

@@ -6,3 +6,4 @@ pbr>=2.0 # Apache-2.0
 cliff>=2.8.0 # Apache-2.0
 osc-lib>=1.7.0 # Apache-2.0
 rsd-lib>=0.0.1 # Apache-2.0
+six>=1.10.0 # MIT

rsdclient/common/utils.py

@@ -36,7 +36,7 @@ def extract_attr(redfish_obj):
     result = {}
     try:
         for key, value in vars(redfish_obj).items():
-            if key == '_path':
+            if key == '_path' and isinstance(value, six.string_types):
                 result['path'] = extract_attr(value)
                 continue
             # Skip all private attributes