api.rst has many stale information now as we have done
many changes in api framework specially removal of
stevedore extensions loading and plain routing.
This commit modify this doc to reflect latest information
and remove v2 specific info which are no longer valid.
Note- in next part we will merge api_2.rst in this doc and
show 'adding api method' step by step.
Change-Id: I0628d8fa0b19c3fb09f1896402fc85dcae90916f
(cherry picked from commit 796f3036c2)
a89f3d55bb