Add Kubectl command executor.

[coollog]

Should work similarly to (and share a lot of code with) Skaffold in #7.

Kubectl should run kubectl commands, including but not limited to:

• kubectl run --dry-run -o yaml
• kubectl logs
• kubectl port-forward

[TadCordle]

Should we add a KubectlDownloader as well? Or are we assuming people already have it installed?

[coollog]

@TadCordle We'll assume (and check) that people have it installed. It could come from a variety of places including downloading the binary releases, building from source, installing via gcloud, etc.

[TadCordle]

A couple other questions: What are the intended use cases for this class? Should it be more generalized and low-level (i.e. you just pass in the kubectl flags and options as strings) or should there be specific method calls for each of our use cases (e.g. one for generating the k8s yaml, one of getting logs, etc.)?

[coollog]

@TadCordle

Use cases:

• Generating kubernetes yaml #18 (kubectl run.../kubectl expose ...)
• Port-forwarding (kubectl port-forward ...)
• Getting logs (kubectl logs ...)
• And others that can easily be added

This class should be more low-level and provide just a wrapper for kubectl including subcommands and flags. The higher-level actions like generating YAML will be the responsibility of other classes.