Accept message as sole argument to NovaException
Accept message as sole argument to NovaException

Fixes bug 860666

Python nose will sometimes recreate an exception to add captured log
information to the string representation of the exception. It assumes
that it can do this by reinstantiating the class of the originally
triggered exception with one argument, a new string description.
NovaException, and some child classes, didn't work well with this
scheme and would result in a further exception during formatting, which
would result in an unformatted string being displayed.

Change-Id: I7152cb7809fa1deb3986c127370f670fb2da342d

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects