Adding SymmetricKey to the Pie object hierarchy

This change adds SymmetricKey to the Pie object hierarchy along with a
test suite and updates to the other classes and test suites composing
the Pie interface.

kmip/tests/unit/pie/objects/test_cryptographic_object.py

@@ -30,6 +30,9 @@ class DummyCryptographicObject(CryptographicObject):
         """
         super(DummyCryptographicObject, self).__init__()
 
+    def validate(self):
+        return
+
     def __repr__(self):
         return ''