Same as commit 8da9aa07, to reduce redundancy.

Was accidentally lost, and documented as the intended value - though it is
not.

Documentation now lives in json schemas.

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.

Only able to use a pre-existing NEO cluster.

Better failing than silently ignoring a missing parameter because of a
typo. Only parameters provided by users should be allowed to be missing,
not internal ones.

This value is used as a directory name, so we should follow the common
denominator.

Also, follow naming convention.

vincent: Minor fixes. Move branch switch to a separate commit.
vincent: Provide configuration to mysql_install_binary.

Loosened checks are considered safe enough, and the speedup on many-GB
FileStorage files is significant.

Split line more naturally (don't separate switch from related argument).
Use long options instead of abbreviations.

Also, reduce code duplication.