Change per-tenant router use-case so tenant uplinks router

The per-tenant router use-case example implies that only the admin user is
is able to set the external gateway on the tenant's router. This is wrong
and this patch changes the example so the tenant does it instead.

Fixes bug: 1233167

Change-Id: Iec82581d3dad145be9d2327b9cf48b290a864f7a
Merge "Change per-tenant router use-case so tenant uplinks router"