erp5: Make "config" macro parameter mandatory.

It's already always provided, and looks dangerous: it is mutated during
execution.
erp5: Publish site-id, as it is auto-generated when not provided.
erp5: Fail instanciating balancer if any family has no known backend.
recipe.erp5_bootstrap: Add support for non-HTTP schemes.

urllib chooses the proper httplib class based on url scheme, so use it.
urllib handles in-url credentials for basic auth, removing the need to
implement it ourselves.
Simplifies URL parsing, uncovering errors on recipe execution rather than
on generated script's execution.
Use %r for automated escaping & quoting.
erp5: Implement automated ERP5Site creation.