Fix credential client to return raw response

Identity v3 credential APIs return credential information
in 'blob' element which is a dict embedded in string.

And service client does json.load on that and then return the
response which should not be done.

Each service client should return the raw response they gets
from API server.

This commit fix the same and move the json loading of nested dict
on test side.

Partially implements blueprint consistent-service-method-names

Related-Bug: 1622806

Change-Id: Ied8c379af2dcb9ab1cbde859f110d218dd9aea5f
4 files changed