Fix Qos Specs association corner case

Fix Qos Specs association corner case

Fix a corner case in associating QoS Specs with Volumt types that was
not handled well: Raise HTTPBadRequest() to client if associate volume
type, which is already associated one qos specs other than specified
qos specs.  Previously such operation will proceed without error which
results overwriting original association.

Also refactor DB API volume_type_qos_specs_get() to return qos specs
in consistent dict format as other qos specs related APIs.

Fix bug: # 1222692

Change-Id: I9db66d1e3a7469620ba542f5387422685a2d828f