Fixing stringent uid value checks in the ProxyKmipClient

This change loosens the input requirements for ProxyKmipClient
operations that accept a secret UID. Operations like Get and
Destroy used to require a string value but are allowed in the
KMIP specification to take no value at all. This change updates
the ProxyKmipClient to properly reflect the specification. The
underlying KMIPProxy client is mostly unchanged.

Closes #261

kmip/services/kmip_client.py

@@ -357,7 +357,7 @@ class KMIPProxy(KMIP):
                             revocation_message=message,
                             credential=credential)
 
-    def destroy(self, uuid, credential=None):
+    def destroy(self, uuid=None, credential=None):
         return self._destroy(unique_identifier=uuid,
                              credential=credential)