Add Decrypt support to the server

This change adds the Decrypt operation to the server. Support is
currently limited to symmetric decryption only. The decryption key
used with the operation must be in the Active state and it must
have the Decrypt bit set in its cryptographic usage mask.

kmip/services/server/crypto/engine.py

@@ -554,7 +554,8 @@ class CryptographyEngine(api.CryptographicEngine):
             plain_text = self._handle_symmetric_padding(
                 self._symmetric_key_algorithms.get(decryption_algorithm),
                 plain_text,
-                padding_method
+                padding_method,
+                undo_padding=True
             )
 
         return plain_text