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"
```
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