Add support to skip stages

[grayside]

The current lifecycle of STS takes the following stages for a sample:

1. Build
2. Deploy
3. Test
4. Teardown

Problem

In working with a tool that owns this whole lifecycle, a few exceptions to the full set of stages often come up:

• In order to minimize time spent troubleshooting later stages of the lifecycle, I may want to skip earlier stages.
• In order to implement tests for an existing container image, I want to skip build.
• In order to troubleshoot failing tests, I want to skip teardown.

Proposed solution

Add the following flags or similar:

• --skip-until: Name the stage you want to run, log the names of stages that are skipped
• --no-teardown: Do not run the teardown process
• --container-image=[IMAGE URI]: If skipping build, developer will need to enter an image URI.

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This issue affects two categories in my review of the repository:

• Adoption: Can this project be found and does it provide enough context on when and how to use it? Does it inspire confidence as a well-maintained project?
• Experience: Does the project provide a good experience to users?
• Scope: Does the tool have the breadth of features and flexibility to support reasonable testing needs?

[SaketramDurbha]

I think this is a great idea.

Only thing is that I'm not sure how parts of this will work with Cloud Build config support (#8) implemented. Because if we say that cloudbuild.yaml files should include build and deploy commands, I'm not sure how we would prevent the building of a new container image when we call gcloud builds submit using that config file. If it's not possible, I think one option is that we can disable skipping the build step in cases where we're using a cloudbuild.yaml file.