Instanciate only if needed

When running the firsts tests on BDD to cover commands, I get an error on redeclaring `eZUpdateDebugSettings`:
```
PHP Fatal error:  Cannot redeclare eZUpdateDebugSettings() (previously declared in /var/www/html/dev/behat/ezpublish_legacy/kernel/private/classes/global_functions.php:91) in /var/www/html/dev/behat/ezpublish_legacy/kernel/classes/ezscript.php on line 1222
```
with stack trace:
```
PHP   1. {main}() /var/www/html/dev/behat/vendor/behat/behat/bin/behat:0
PHP   2. Symfony\Component\Console\Application->run() /var/www/html/dev/behat/vendor/behat/behat/bin/behat:32
PHP   3. Behat\Behat\Console\BehatApplication->doRun() /var/www/html/dev/behat/vendor/symfony/symfony/src/Symfony/Component/Console/Application.php:121
PHP   4. Symfony\Component\Console\Application->doRun() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Console/BehatApplication.php:68
PHP   5. Symfony\Component\Console\Application->doRunCommand() /var/www/html/dev/behat/vendor/symfony/symfony/src/Symfony/Component/Console/Application.php:191
PHP   6. Symfony\Component\Console\Command\Command->run() /var/www/html/dev/behat/vendor/symfony/symfony/src/Symfony/Component/Console/Application.php:901
PHP   7. Behat\Behat\Console\Command\BehatCommand->execute() /var/www/html/dev/behat/vendor/symfony/symfony/src/Symfony/Component/Console/Command/Command.php:244
PHP   8. Behat\Behat\Console\Command\BehatCommand->runFeatures() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Console/Command/BehatCommand.php:128
PHP   9. Behat\Gherkin\Node\AbstractNode->accept() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Console/Command/BehatCommand.php:150
PHP  10. Behat\Behat\Tester\FeatureTester->visit() /var/www/html/dev/behat/vendor/behat/gherkin/src/Behat/Gherkin/Node/AbstractNode.php:42
PHP  11. Behat\Gherkin\Node\AbstractNode->accept() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Tester/FeatureTester.php:88
PHP  12. Behat\Behat\Tester\ScenarioTester->visit() /var/www/html/dev/behat/vendor/behat/gherkin/src/Behat/Gherkin/Node/AbstractNode.php:42
PHP  13. Behat\Behat\Tester\ScenarioTester->visitStep() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Tester/ScenarioTester.php:87
PHP  14. Behat\Gherkin\Node\AbstractNode->accept() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Tester/ScenarioTester.php:148
PHP  15. Behat\Behat\Tester\StepTester->visit() /var/www/html/dev/behat/vendor/behat/gherkin/src/Behat/Gherkin/Node/AbstractNode.php:42
PHP  16. Behat\Behat\Tester\StepTester->executeStep() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Tester/StepTester.php:95
PHP  17. Behat\Behat\Tester\StepTester->executeStepDefinition() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Tester/StepTester.php:126
PHP  18. Behat\Behat\Definition\Annotation\Definition->run() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Tester/StepTester.php:157
PHP  19. call_user_func_array:{/var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Definition/Annotation/Definition.php:155}() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Definition/Annotation/Definition.php:155
PHP  20. EzSystems\BehatBundle\Context\CommandContext->iExecuteCommandWith() /var/www/html/dev/behat/vendor/behat/behat/src/Behat/Behat/Definition/Annotation/Definition.php:155
PHP  21. Symfony\Component\Console\Tester\CommandTester->execute() /var/www/html/dev/behat/vendor/ezsystems/behatbundle/EzSystems/BehatBundle/Context/CommandContext.php:63
PHP  22. Symfony\Component\Console\Command\Command->run() /var/www/html/dev/behat/vendor/symfony/symfony/src/Symfony/Component/Console/Tester/CommandTester.php:70
PHP  23. Symfony\Bundle\FrameworkBundle\Command\CacheClearCommand->execute() /var/www/html/dev/behat/vendor/symfony/symfony/src/Symfony/Component/Console/Command/Command.php:244
PHP  24. Symfony\Component\HttpKernel\CacheClearer\ChainCacheClearer->clear() /var/www/html/dev/behat/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Command/CacheClearCommand.php:70
PHP  25. eZ\Bundle\EzPublishLegacyBundle\Cache\LegacyCachePurger->clear() /var/www/html/dev/behat/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/CacheClearer/ChainCacheClearer.php:42
PHP  26. eZ\Publish\Core\MVC\Legacy\Kernel->runCallback() /var/www/html/dev/behat/vendor/ezsystems/ezpublish-kernel/eZ/Bundle/EzPublishLegacyBundle/Cache/LegacyCachePurger.php:83
PHP  27. ezpKernel->runCallback() /var/www/html/dev/behat/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/MVC/Legacy/Kernel.php:162
PHP  28. ezpKernelWeb->runCallback() /var/www/html/dev/behat/ezpublish_legacy/kernel/private/classes/ezpkernel.php:68
PHP  29. eZ\Bundle\EzPublishLegacyBundle\Cache\{closure}() /var/www/html/dev/behat/ezpublish_legacy/kernel/private/classes/ezpkernelweb.php:1209
PHP  30. ezpAutoloader::autoload() /var/www/html/dev/behat/ezpublish_legacy/kernel/private/classes/ezpkernelweb.php:0
```

This solves the error no sure if it's fixing the problem or an symptom.