Add a unit test to check initializers of MistralException hierarcy
Add a unit test to check initializers of MistralException hierarcy

* This test checks that all classes derived from MistralException
  have one initializer with only one "message" parameter. It's
  because at some places (rpc/base.py at least) we assume that
  such an initializer exists.

Change-Id: Ieb1cf1510d88553038cd812d87fb7698755406e7

openstack/mistral,openstack/mistral