Create swift operator keystone roles

Defining operator_roles in swift-proxy.conf requires
that these roles exist in keystone.

Because Keystone API access is not available on Swift
proxy hosts in most cases, operator_roles keystone
roles have been added to swift::keystone::auth class.

Added basic docs for swift::keystone::auth

Change-Id: Ib8454f8ae1316d2589ff228ec1424c6cff69c6b5
Closes-Bug: #1261662
(cherry picked from commit 14a2bd8c4fdad27fe708792c8a040c55a7476c56)
Merge "Create swift operator keystone roles" into stable/havana