Rename using-api.rst to using-api-v2.rst

Renaming file for consistency, since using-api-v3.rst is being written
there: https://review.openstack.org/#/c/63408

Minor changes have been made to indicate that the file documents the
keystone v2 API.

Change-Id: I694b658a8b59d21615af5d88edc0f7b394ebbe7b
Partial-Bug: #1260527
Merge "Rename using-api.rst to using-api-v2.rst"