Include credentials for heat calling self

When in standalone mode, heat needs to include heat credentials
when calling its own REST API.

This change assumes that heat is running in standalone mode if
heat.conf [clients_heat] url has been set.

Change-Id: I34e662c554ea66c6df6ebee6ce3bfb0dc3f8a948
Closes-Bug: #1302578
Merge "Include credentials for heat calling self"