Make Message keep string interpolation args

Update the Message class handling of __mod__() so that arguments
from previous calls are retained. This works around an issue
found in neutron, and likely present in other parts of OpenStack,
where exception classes are performing string interpolation on
Message objects more than one time before emitting the translated
results. See the test comments for an example.

Change-Id: I4c72bb255af7d0a120b702707e11361fedce0f24
Fixes-bug: #1270212
Merge "Make Message keep string interpolation args"