use the root path property of the base admin

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

**Depends on https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/issues/167**

Use the root path provided by the base admin.