Refactoring: Service::getInstanceName()

Summary:
The config:show command would like a string to represent the instance
in an array. We transfer the responsability to provide this string to
the Service class.

Test Plan:

• A new test is provided to test the method in the two cases
• php artisan config:show

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D302