action_log creation is failed on requests for node assignment

[Artem Roma]

Code of CollectionMonitorMiddleware class fails while processing requests for node assignment/unassignment which leads to failed action log creation and occurring of unhandled exception in middleware stack. This is happening because corresponding handlers return response in format that are not JSON compatible.

To fix this one must rewrite handlers' code that they return data in proper format.

Code that fails inside of middleware class:
https://github.com/stackforge/fuel-web/blob/master/nailgun/nailgun/middleware/connection_monitor.py#L195

Handlers to be fixed:
https://github.com/stackforge/fuel-web/blob/master/nailgun/nailgun/api/v1/handlers/assignment.py

[Artem Roma (aroma-x)]

Code of connection monitor middleware has been changed so in case if we collect action logs for nodes assignment it won't fail. For that reason move bug's status to invalid.