security acl dbal schema: inject the schema instead of the whole container

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

following a discussion with @Stof on https://github.com/doctrine/DoctrinePHPCRBundle/pull/78 i thought to clean up the code i used as template as well.