CinderException args to strings when exceptions

CinderException message and kwargs are converted to strings if the
argument is itself an exception.

Exceptions passed as arguments to CinderException happen in many areas
of the code. This causes an error when oslo.messaging tries to serialise
the exception.

Change-Id: I1399c939bbca47ab8362ac3bbe0e9a349c6d5572
Closes-Bug: #1301249
Merge "CinderException args to strings when exceptions"