Fix discover command failed to read extension list issue

Fix the key error which caused discover command failed to read the
response body of extension list result. This change also added test
cases to cover the use case of generic client extension discovery.

Change-Id: Id687f8d73cead28f594de00d3b5ff9086558947b
Closes-Bug: #1266710
Merge "Fix discover command failed to read extension list issue"