Custom images using packer (#283)

- Added new directory: builds/packer
- Created opinionated images for ocp-clientvm (outside aad) and enable AAD to use it
- Define workflow, add Readme.adoc
- Finding the opinionated AMI is dynamic: use tags on the images to identify which one to use
- Update ec2 Cloud template to pick 'custom_image' if it's available
- Fallback to 'repo_version' instead of 'osrelease' when detecting

Allow this simple use-case:

- Create a common RHEL gold images and enable AAD to use it

When deploying, user only provides the name of the image to look for.

- If the image exists: use it
- If the image does not exist: default to CF template

'custom_image_filter' is a lower priority than 'custom_image_stage' which detects opinionated
images that are specific to the tuple (env_type, version, stage).

- Enable custom_image in default Cloudformation template
- Ensure ec2_ami_facts is succeeded before checking the register
- Use another register for the second call ec2_ami_facts to avoid bugs
- Condition hardening