Automatic merge from submit-queue (batch tested with PRs 40111, 40368, 40342, 40274, 39443)

Change OPENSTACK_IMAGE_NAME to be more specific

There may already be other images in the cloud named CentOS7,
but since we are fetching a very specific version (1604) we should
go ahead and make the image name very specific as well.



**What this PR does / why we need it**:

Some clouds already have `Centos7` as an image that is available, however it may not be the *specific* version that openstack-heat looks for and downloads from CentOS.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: 

**Special notes for your reviewer**:

**Release note**:

```release-note 
OpenStack-Heat will now look for an image named "CentOS-7-x86_64-GenericCloud-1604". To restore the previous behavior set OPENSTACK_IMAGE_NAME="CentOS7"
```