Add DeleteAttribute support to the client

This change adds DeleteAttribute support to the ProxyKmipClient,
leveraging the new generic request capability in the underlying
KMIPProxy client. Going forward all new attribute support will
leverage the new request capability and older supported operations
will be migrated to use it as well, with the ultimate vision
being a final merger of the two client classes into one easy to
use architecture. New unit tests have been added to cover the new
client additions.

Partially implements #547

kmip/core/exceptions.py

@@ -228,6 +228,23 @@ class KeyFormatTypeNotSupported(KmipError):
         )
 
 
+class OperationFailure(KmipError):
+    """
+    An exception raised upon the failure of a KMIP appliance operation.
+    """
+    def __init__(self, status, reason, message):
+        """
+        Construct the error message and attributes for the KMIP operation
+        failure.
+
+        Args:
+            status: a ResultStatus enumeration
+            reason: a ResultReason enumeration
+            message: a string providing additional error information
+        """
+        super(OperationFailure, self).__init__(status, reason, message)
+
+
 class OperationNotSupported(KmipError):
     """
     An error generated when an unsupported operation is invoked.