Extensions API

    1. Added an extensions-api package to Identity.
    2. Moved Identity base and constant models to a common package.
    2. Added extensions-api responses model and client.
    3. Added a data folder for extensions-api tests.
    4. Tested extensions-api extension responses model and client.
    5. Fixed pep8 issues as per reviewer comments.
    6. Cherry-pick commit: 9c3806200a310d75f2643cdb57631a608c85a9ef.
    7. Added extensions-api admin parameter to extensions-api client.
    8. Added and tested list roles to extensions-api client.
    9. Made a small fix (import of Roles).
    10. Renamed sections and properties in Identity config.py.
    11. Renamed client to ExtensionsAPI_Client for consistency.
    12. Fixed all reviewer comments.
    13. Added a create role functionality to extensions-api client.
    14. Added create role test and refactored extensions-api client test.
    15. Added and tested delete role functionality to extensions-api client.
    16. Fixed reviewer comments - line continuation issues.
    17. Fixed reviewer comments - refactored extensions-api client and its test.
    18. Removed a section and property (extensions-api-admin) from Identity config.py.
    19. Added an admin extensions class to the constants model, to use it across API clients.
    20. Refactored extensions-api client and its test.
    21. Rebased changes and uploaded a new patchset.

Change-Id: I4f0691b5dfd2302c3414b3c8d4b881b1624063e6
Merge "Extensions API"