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
Sync with global requirements

Change-Id: I366b3a23bcb82203b347fa13f0385024bae62cab
Fix E12x warnings found by Pep8 1.4.6

These are missed due to bugs in 1.4.5.

Closes-Bug: #1267994
Change-Id: Ic7b37abf757a9b7b0c8306c941949dee93b83e08
Merge "Fix E12x warnings found by Pep8 1.4.6"
Merge "Fix discover command failed to read extension list issue"
Merge "Sync with global requirements"