Resolve circular import, introduce ExecuteCommandMixin

Adding new methods to utils won't work as the circular import appears
between errors.py and utils.py.

Introduce ExecuteCommandMixin and use it for IronicPythonAgent and
FlowExtension (in future patch).

Also add tests for its.

Change-Id: Id95b31349292a7967d2ee66ec82c1662d8e5de94