API Extensions framework for v3 API Part 2

This is the second patch for the new extension framework
which is only to be used by the Nova v3 API.

- Adds tracking of extensions loaded and allows
  extensions access to this information
- Adds core API functionality as extensions
  - 'server'
    - Adds an entry point that other extensions can
      use to modify the server create arguments without
      having to modify the server extension itself
    - TODO: Will have to add more entry points as other
      extensions are ported. Delaying adding entry points
      now so they can be tested as they are added.
- Adds port of os-keypairs extension
  - This is an example of a controller extension in the new
    framework
  - This is an example of using the server extension entry
    point to add functionality without modify the core API code
  - Ports tests for the os-keypairs extensions
    - Adds v3 API fake specific code for tests

This completes the bulk of the new extension framework. Porting
of the server tests will be done in future changesets as more
of the core API is ported across as the tests are dependent
on multiple core APIs.

Partially implements blueprint v3-api-extension-framework

Change-Id: Ibadb5bbe808c27d2f4afebe65c06a92576397085
Merge "API Extensions framework for v3 API Part 2"