Post Processor Glance / Openstack

_This issue was originally opened by @imduffy15 as hashicorp/packer#2678. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

Hey All,

I've gone through some of the past pull requests and issues, I've found two around the topic of uploading images to openstack/glance directly from packer:
- https://github.com/mitchellh/packer/issues/540
- https://github.com/mitchellh/packer/pull/2410

Neither of these have been merged though. I have started writing my own one outside of packer and would love to get feedback, improve it, and eventually merge it in with master.

I have extended gophercloud to support the creation of images and the uploads of their binaries:
- https://github.com/monkeylittleinc/gophercloud/pull/1/files

Using this extended gophercloud I've built out a packer plugin to upload a qcow2 or raw file outputted by qemu up
- https://github.com/monkeylittleinc/packer-post-processor-glance/tree/iduffy-dev

I'm currently working on tests... Got any recommendations/examples on how to mock up the openstack service client? I took a brief look at other tests and they just seemed to avoid talking to external sources.

Thanks,
Ian.