bug symfony#16734 Make sure security.role_hierarchy.roles always exists (WouterJ)

This PR was squashed before being merged into the 2.8 branch (closes symfony#16734).

Discussion
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Make sure security.role_hierarchy.roles always exists

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#16731
| License       | MIT
| Doc PR        | -

Commits
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b272ab5 Make sure security.role_hierarchy.roles always exists

Author: fabpot

src/Symfony/Bundle/SecurityBundle/Resources/config/security.xml

@@ -47,6 +47,7 @@
         <parameter key="security.validator.user_password.class">Symfony\Component\Security\Core\Validator\Constraints\UserPasswordValidator</parameter>
 
         <parameter key="security.expression_language.class">Symfony\Component\Security\Core\Authorization\ExpressionLanguage</parameter>
+        <parameter key="security.role_hierarchy.roles" type="collection" />
     </parameters>
 
     <services>

src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/SecurityExtensionTest.php

@@ -115,10 +115,7 @@ public function testDisableRoleHierarchyVoter()
 
         $container->compile();
 
-        $admDefinition = $container->getDefinition('security.access.decision_manager');
-        $registeredVoters = array_map('strval', $admDefinition->getArgument(0));
-
-        $this->assertNotContains('security.access.role_hierarchy_voter', $registeredVoters);
+        $this->assertFalse($container->hasDefinition('security.access.role_hierarchy_voter'));
     }
 
     protected function getRawContainer()