Cleaned up some TODO comments for feature/qos that do not apply anymore

Cleaned up some TODO comments for feature/qos that do not apply anymore

1. Removed comment to add tests for invalid input for attach/detach
methods for QosPolicy. Those tests are already implemented in the test
class, so the TODO became obsolete.

2. Removed TODO to use a constant for rule 'type' field. There is no
'type' field in rules anymore, so it does not apply any more.

Change-Id: I205cbc2d9a2eeee1a0a9fb5794efc063de6f326d
Partially-Implements: blueprint quantum-qos-api