[Queue\Capsule] Only create a "config" instance if only it not yet created.

| Q             | A
| ------------- | ---
| Bug fix?      | Yes (in some use case where you use illuminate/config with illuminate/queue
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | None
| License       | MIT
| Doc PR        | None

Make it consistent with changes to Illuminate\Database\Capsule\Manager
merged in laravel/framework#4598

Signed-off-by: crynobone <crynobone@gmail.com>