Previously cinder list_extensions test was disabled (by starting
the test name with an underscore) and REST API for cinder extensions
did not work properly.
This commit fixes the REST API and corresponding unit test.
To make it easy to write the test, test data for list_extensions
response of cinder v2 API is also added.
Related-Bug: #1752917
Change-Id: I5ac014d62462529e186e9dde3842c6a8a629f1c4