py3: Fix usage of JSON in API contrib

py3: Fix usage of JSON in API contrib

Replace json.dumps() and jsonutils.dumps() with
jsonutils.dump_as_bytes() to get bytes on Python 3. The result is
used as HTTP body, and the body type must be bytes.

Replace also json.loads() with jsonutils.loads().

Partial-Implements: blueprint cinder-python3
Change-Id: I3d3a18f2013b42dc2d8d7b90f32f50756513d668