[DIC] Better handling of enableable configurations

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no, this feature has not been released yet
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

My definition of bug fix might be discussable. The thing which I think is not discussable is that this PR fixes the semantic - and I think it is important for a "semantic configuration": before this PR, some nodes had `->canBeDisabled` for nodes that were actually disabled by default. Those nodes now have `->canBeEnabled` which sounds right.

**Edit: Jan 28, 2013** - history: 
 
See [the related comments](https://github.com/symfony/symfony/pull/6829#discussion_r2727742).

I think Symfony **must** get the configuration right as we can expect of lot of devs to use this as a template when writting their own configuration.

@schmittjoh could you please give me your feedback on [this change](https://github.com/symfony/symfony/pull/6852/files#L4R224) considering [the rationale](https://github.com/symfony/symfony/pull/6852/files#L3R7).