Automatic merge from submit-queue (batch tested with PRs 40978, 40994, 41008, 40622)

Refactored kubemark code into provider-specific and provider-independent parts [Part-2]

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:

- Added test/kubemark/skeleton/util.sh which defines a well-commented interface that any cloud-provider should implement to run kubemark.
  This includes functions like creating the master machine instance along with its resources, remotely executing a given command on the master (like ssh), scp, deleting the master instance and its resources.
  All these functions have to be over-ridden by each cloud provider inside the file /test/kubemark/$CLOUD_PROVIDER/util.sh
- Implemented the above mentioned interface for gce in /test/kubemark/$CLOUD_PROVIDER/util.sh
- Made start- and stop- kubemark scripts (almost) provider independent by making them source the interface based on cloud provider.

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