Doc - Keystone configuration - moving RBAC section

Moved the `Keystone API protection with RBAC` section between sections
`Configuration files` and `Adding Extensions`. It makes sense, since
it documents the `policy.json` file. This section was previously a
subsection of `Adding Users, Tenants, and Roles with
python-keystoneclient`.

Change-Id: I554f357c40666e67356e176ff82d057894e8bfe5
Merge "Doc - Keystone configuration - moving RBAC section"