Extend documentation to cover rhel-edge-installation composer-cli restriction on blueprints

[jordigilh]

The documentation does not mention that when building an ostree with composer-cli for rhel-edge-installer, it is not possible to use a populated blueprint. The documentation at the RHEL site states that:

The “RHEL for Edge Installer (.iso)” image type does not support blueprint customizations or package selection. Therefore, if any customization or packages are specified, the build fails. 

This problem arises when running the rfe-oci-iso pipeline, which executes a composer-cli command like this:

$>  composer-cli compose start-ostree --ref rhel/8/x86_64/edge --url http://httpd-rfe.apps.mycluster.redhat.com/microshift/latest microshift  rhel-edge-installer
2022-01-11 09:14:14,783: failed to create osbuild manifest: boot ISO image type "edge-installer" does not support blueprint customizations

[sabre1041]

My recommendation is that we add logic in the ISO creation pipeline to disable non default sources, perform the build and then reenable once complete.

Thoughts @jordigilh @nasx ?

[jordigilh]

Why do we need to re-enable them after the build is completed? Why not leave them disabled by default?

[sabre1041]

@jordigilh they need to be reenabled to support subsequent builds