Improvements of Heat stack update

Hi,

This patch follows the previously merged patch for the initial import. This patch adds:

* Support for the 'overcloud' Heat stack, that we assume is already created.
* Support for updating this Heat stack:

```
curl -XPOST -H 'Content-Type:application/json' -H 'Accept: application/json' http://0.0.0.0.0:6385/v1/data_centers/
```

Issuing this command, the overcloud Heat template will be updated and the Racks will be registered from all ResourceClasses

* The 'Rack' state will change to 'provisioned' once the Rack is properly registered in Heat