[DoctrineBridgeBundle] Support of non default entity manager for EntityUserProvider

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | doctrine/DoctrineBundle#188
| License       | MIT

this is reference to doctrine/DoctrineBundle#188

TLDR The EntityUserProvider found in DoctrineBridgeBundle only assumes entity to use default EntityManager.