Automatic merge from submit-queue (batch tested with PRs 39469, 40557)

Refactored kubemark code into provider-specific and provider-independent parts [Part-1]

Applying part of the changes of PR https://github.com/kubernetes/kubernetes/pull/39033 (which refactored kubemark code completely). The changes included in this PR are:

The following are the major changes as part of this refactoring:
- Moved cluster-kubemark/config-default.sh -> cluster-kubemark/gce/config-default.sh (as the config is gce-specific)
- Changed kubernetes/cluster/kubemark/util.sh to source the right scripts based on the cloud-provider
- Added the file test/kubemark/cloud-provider-config.sh which sets the variable CLOUD_PROVIDER that is later picked up by various scripts (run-e2e-tests.sh, common.sh)
- Removed useless code and restructured start-kubemark.sh and stop-kubemark.sh scripts.

@kubernetes/sig-scalability-misc @wojtek-t @gmarek