Update curl api example to specify tenant

In the keystone api_curl_examples document, the curl example for
"POST /tokens" did not specify a tenant. This example utilized
the default tenant, which is not commonly used. Changed the
example to include "tenantName" as part of the example to prevent
future confusion.

DocImpact

Closes-Bug: #1269739

Change-Id: Iaad8c7c5a32bc6cff38c938dd36365cddd6741cb
Merge "Update curl api example to specify tenant"