Use skaffold modules & update relevant docs

[NimJay]

Fixes #492.

Change summary:
This pull-request:

• splits the existing contents of skaffold.yaml into two modules: app; and loadgenerator.
  • We can now run something like skaffold run ... --module=app to run this sample app without the loadbalancer.
• updates docs accordingly (see this comment from Bank of Anthos that confirms that we need skaffold 1.27+).

Additional Notes

• Running skaffold run or skaffold dev will still behave the same as it did before (i.e., run all microservices).
• Unlike with Bank of Anthos, we don't have modules for backend, frontend, setup, and db.
  • The reason: I don't imagine anyone wanting to run just the front-end or just the back-end (for this sample app). But I do imagine someone wanting to run this sample app without the loadbalancer.
  • Plus, I think we only really need 2 modules to showcase this skaffold feature.
  • I am very open to changing this, so please provide suggestions. :)

Tested 👍
I have tested the following commands and things work as expected:

• skaffold run --default-repo=gcr.io/nimjay-microservices-demo
• skaffold run --default-repo=gcr.io/nimjay-microservices-demo --module=app
• skaffold run --default-repo=gcr.io/nimjay-microservices-demo --module=loadgenerator

Failing Check ⚠️
A pull-request Check is currently failing because I have yet to upgrade the version of skaffold on our virtual machines (similar to Bank of Anthos).

[NimJay]

Hi @gsquared94,
I am not able to assign you as a reviewer.
But a quick review from you would be extremely useful! :)
Let me know if you'd like me to change anything.

[gsquared94]

lgtm!

[askmeegs]

Upgraded GitHub actions runners from skaffold 1.21 to skaffold latest (1.33).

[github-actions]

🚲 PR staged at http://34.138.23.95