Allow not to request instance in a test.

This adds the request_instance class variable to SlapOSInstanceTestCase. If this variable is false, no instance will be requested.

I need this feature in order to create tests for components that are not supposed to have a software release.

Note: As I needed to add a new class variable, I tried to follow the "Boy Scout rule" and clean a bit the class variables (in a separate commit). If that is not appropriate, or I did not made the right choices, please tell me.