Get rid of unused/unsupported software type.

Drop instance-erp5-single and instance-zope:
All deployments of ERP5 instances must be covered by cluster-style
deployment, and tweaked by parameters rather than software type.
Drop instance-varnish:
This is not used currently, so avoid giving the impression that it is
supported. Revive when properly integrated.
Get rid of single/cluster distinction last traces.
Enable zeo backups by default, to be consistent with mariadb.
Do not instanciate cloudooo.

Cloudooo should be instanciated separately when needed, using its
dedicated software release.
Allows getting rid of a few large builds (parts of xorg, libreoffice),
reducing needed disk space.
Also, get rid of a few more unused components.
Also, move unused-but-kept components together.
Switch branch.