fix root path to use a unique name instead of hiding admin root as defined by base sonata admin

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes (currently used in SimpleCmsBundle)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

This was duplicated in all the bundles providing admins, except for SimpleCms. The name root is already used by base sonata admin.