runtest: Add support for alias image URLs

[jharuda]

It attempts to add functionality to get the latest image from a URL, where URL (source field in the config) is a alias for the latest image. It is useful for CentOS images. It uses the metadata of the web resource to distinguish whether a new image needs to be downloaded.

[jharuda]

Ideas for what can be done differently:

• I can remove checking origurl != URL because we use last modified date
• I can add a new optional option in the config.json and call it alias_url. It will have the similar functionality as source, but in the code it will use last_modified attribute comparison. If there is no alias_url option in the config, it will use the current comparison ( (origurl != URL).

[pcahyna]

I don't understand from the description what is it supposed to do. Can you provide an example? Also, is it a new feature or does it fix a misbehavior in some cases?

[jharuda]

@pcahyna It is a new feature for selecting CentOS images. Currently we support 2 methods how to define/find image: 1) compose URL and 2) source URL. When we use the source URL method, we expect that the uploaded image will not be changed and(or) updated. The point of this PR is to add support for third method. In the third method I extended source URL functionality to support alias URL (by alias URL I mean for example https://cloud.centos.org/centos/7/images/CentOS-7-x86_64-GenericCloud.qcow2c). The third method is useful because we will not need to manually update the CentOS images in the config to CentOS-7-x86_64-GenericCloud-1907.qcow2c, then to the CentOS-7-x86_64-GenericCloud-2003.qcow2c then to the CentOS-7-x86_64-GenericCloud-2009.qcow2c ... every time there is a new CentOS image.

[richm]

please fix the tox issues

[jharuda]

please fix the tox issues

done