AII schema should not be extensible

[jouvin]

Up to Quattor 16.6.0, AII schema (/system/aii/xxx) is making an extensive use of extensible. Basically all parts of the schema are extensible and only some parts are bounded to a real schema. As a result, every typo or mistake in configuration paths cannot be catched during compilation and result in aii-shellfe errors not always easy to understand (see #200 for example)...

The motivation for this is probably to allow an easy addition of hooks in various parts of AII. It would be good to find a way to provide the flexibility of hooks without loosing the ability to validate the configuration.

[jrha]

Discussed at workshop and agreed to survey the community for hook usage and remove the extensible option.

[jrha]

Found more hooks!, See #245.

[jrha]

Discussed at March 2017 workshop, agreed that:

1. structure_aii should not be extensible.
2. structure_aii_hooklist will need to remain extensible unless it can be restructured in future.
3. structure_aii_module shoud be removed and proper types defined for each module.

[jrha]

New implementation of aii_commands_hook happening in #349.