Improve cinder CLI existing tests

Objective of this patch to parse the output of read only cinder CLI's,
to validate the output and improve more output verification.
It is improving the code of all below read only cinder CLI's subcommands:
1. backup-list
2. extra-specs-list
3. list
4. rate-limits
5. snapshot-list
6. type-list
7. credentials
8. availability-zone-list
9. endpoints
10. service-list
11. transfer-list
12. qos-list
13. encryption-type-list

Partially implements: blueprint cli-tests-coverage

Change-Id: I3fbbc44194898b9a699a9807a7a800e9a9cb925d
Merge "Improve cinder CLI existing tests"