Add support for controlling amount of nodes

What does this MR do?

Adds support for controlling the amount of nodes that our QA::Service::KubernetesCluster spins up.

By default, this number is '3'. If you have 3 replications, that number can turn into 9 nodes.

When our QA autodevops spec runs each time, it spins up 9 nodes.

TODO:

• Add GCLOUD_NUM_NODES=1 to our CI/CD

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

This not only should take less time to spin up, therefore the test runs faster, but this should also ease costs.