Introduce custom_image_filter variable

Allow this simple usecase:
- create a common RHEL gold images and use it

When deploying, user only provides the name of 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 detect
images that are specific to the tuple (env_type, version, stage).