Only create a "config" instance if only it not yet created.

Make it consistent with changes to Illuminate\Database\Capsule\Manager
merged in laravel/framework#4598

Signed-off-by: crynobone <crynobone@gmail.com>
Merge pull request #5062 from crynobone/feature/capsule-queue-config

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