Extract Virtual Machine Configuration from GitLab Service Node Definition

This issued started with a discussion started by @ghickey from the initial terraform work for multiple clusters.

As we discussed yesterday, I really think sending a "complex" data structure to a module is the wrong approach. It limits the reusability of the module by others and other projects. Certainly to keep things moving forward proceed the way it is and I will get an issue created so that this can be further discussed.

Abstracting the virtual machine allows us to start working on defining more abstract notions such as what constitutes small, medium and large resources](#141).

Complications:

• modules do not yet support count in terraform

The goal for this issue is to separate the logic that defines resources in GCP from the logic that creates a GitLab service node.