This commit associates `system` to revoke event policies, since these
policies were developed to assist the system in offline token
From now on, a warning will be logged when a project-scoped token is
used to get revocation events. Operators can opt into requiring
system-scoped tokens for these policies by enabling oslo.policy's
`enforce_scope` configuration option, which will result in an
HTTP Forbidden exception when mismatching scope is used.