Add "get extensions" calls to all APIs

Closes-Bug: #1314302

Change-Id: I4acd837002749c2214d70bb7dc1cc2560ac08024
author: diane fleming
Merge "Add "get extensions" calls to all APIs"
Add server groups extension to compute v2

Closes-Bug: #1302202

Change-Id: I3f48187485207b615e9cc9eebc1a6f7c2aaf08a6
author: diane fleming