Add support for extension list

 - Add support in the common section for extension list. This only
   supports Identity for now. Once the APIs for volume and compute
   are supported in the respective APIs, they will be added. Once
   network is added to this client, it will be added (the API already
   supports it).
 - Include extension fakes for volume and compute for pre-enablement.

Change-Id: Iebb0156a779887d2ab06488a2a27b70b56369376
Closes-Bug: #1319115
Merge "Add support for extension list"