Introduce instance-base to avoid having PROD instances inherit from instance0 (maintenance)

[lukasgraf]

We want to introduce something like an [instance-base] part that acts as an "abstract base class" for defaults that are common to both production (1-4) and maintenance (0) instances.

This is needed because we want to be able to add maintenance-instance-specific settings to instance0 (like, no warmup, stop trying to connect to ZEO after x seconds and exit) without running the risk of these settings falling through to production instances.

This would allow us to solve #59 cleanly.

Once we perform this change, we need to:

• Identify all buildout configurations from all our repositories
• Check each of these buildouts for custom [instance0] modifications
• Rewrite all those [instance0] modifications to [instance-base]

[jone]

👍 sounds good