Add flag to disable test framework resource creation

[AlexNPavel]

Feature Request

This is a feature request that came from the CoreOS/Redhat slack.

Is your feature request related to a problem? Please describe.
It would be nice to have a way to use a running operator that already exists in the cluster to run the tests. Currently, global and namespaced resources get created by the operator-sdk's test framework, and there are some situations where this is not ideal.

Describe the solution you'd like
We can add a flag (for example --no-setup) that disables resource generation. This should be a simple solution, as we should be able to pass an empty manifest file to stop resources from being created.

cc @aravindhp