Base infrastructure for QoS API tests

Base infrastructure for QoS API tests

This introduces the basic methods in the tempest client, that allow the
testing of the QoS plugin. This also contains 2 (very) simple tests
which test creation and deletion of both policies and bandwidth rules,
as well as list/show for both resources. While creation is done
explicitly, deletion is done implicitly (all resources are deleted after
the test, during tearDown)

Minor fixes to the QoS plugin are included as well.

Change-Id: I0f34ed8464857859bcd519e301a49b0b067593b0