docs(samples): add usage samples to show handling of LRO response Operation

[Shabirmean]

Fixes #3

Description

• For a detailed description read internal ref b/189985582
• The Kubernetes engine API protos were written before the API Improvement Proposal (AIP-151)
• AIP-151 suggests to annotate API calls that takes a long time to complete with a certain annotation. This will enable the auto-generator (that generates our client libraries) to wrap the response an object of type google.longrunning.Operation in a language native (Python Future or Nodejs Promise) async construct.
• However, since AIP-151 came long after the container.v1 API protos were written, the container API protos don't have the necessary annotation as guided in AIP-151
• As a result the auto generated container client libraries do not return the very useful google.longrunning.Operation object, instead they return google.container.v1.Operation
• This object does not have the necessary built-ins to easily verify if the operation has completed.
• Thus this PR adds a few samples to show to customers how they can use this Operation object to validate if their operation has successfully completed using polling with backoff on the getOperation() API

Changes

• Added 3 samples to showcase usage of the library and to show how to handle responses of type google.container.v1.Operation
  • A quickstart sample which simply lists the clusters
  • A create cluster and delete cluster example that returns an Operation object since they are long running operations
• Updated the READMEs explain the samples
• I have also added test cases for the added samples

Testing

• Create a project
• Enable the container (GKE) API
• Create a service account with permissions to create/delete clusters (can use Owner)
• Download a key for the created service account

export GOOGLE_APPLICATION_CREDENTIALS="<PATH_TO_KEY>"
gcloud config set project <GCP_PROJECT>

cd <repo_path>
git checkout lro-fix
cd samples/snippets

python quickstart.js <GCP_PROJECT> us-central1-b
python create_cluster.js <GCP_PROJECT> us-central1-b gke10
python delete_cluster.js <GCP_PROJECT> us-central1-b gke10

nox -s py-3.6

[Shabirmean]

@busunkim96 - adding you in here since you had been part of the discussion in #3 and also b/189985582. Can you TAL and see if it sufficiently addresses the requirements?

[Shabirmean]

@parthea - I've addressed the comments we discussed offline:

• Remove inline code for the landing page README
• Add region tags
• Add tests

Please TAL look when you have some time! Thank you 🙌🏼