Background:
For every release an update of the image name is needed. Currently, image names are updated manually. As an example, some steps of the documentation refer to
registry.access.redhat.com/3scale-amp24/apicast-gateway
instead of
registry.redhat.io/3scale-amp25/apicast-gateway

Goals:

1. Include a new attribute in document-attributes.adoc, called ImageName, which will include the image name of the current release.
2. Review the entire docs and replace the image names with the attribute, as we did with product names and versions for URLs.