EZP-20426 - Legacy cronjobs and scripts should be launchable from Symfony CLI

**Work in progress, DO NOT MERGE**

Ref https://jira.ez.no/browse/EZP-20426.
Related PR : https://github.com/ezsystems/ezpublish-kernel/pull/229

## Tasks
* [x] Refactoring of eZScript allowing to update settings while recalling the singleton
* [x] Avoid `ezpAutoloader` class to be declared twice