Make documentation consistent with other plugins

[stephenashank]

Example: https://github.com/jenkinsci/google-kubernetes-engine-plugin/blob/develop/docs/Home.md#iam-credentials

Also good to reference: https://cloud.google.com/iam/docs/creating-managing-service-accounts

[FridayPush]

On that note, the documentation says 'for how to consume this plugin go look at our cloud storage plugin'
https://github.com/jenkinsci/google-oauth-plugin/blob/develop/docs/home.md

But there's not example there of how they consume it. Unless maybe you are familiar enough with the plugin directory structure to know where they're actually accessing the credentials.

[stephenashank]

Thanks for pointing that out, we'll definitely be updating this part of the docs as well.

As for what it's referring to there, the place it is consumed is in UploadModule. The two types of GoogleRobotCredentials that this plugin provides (GoogleRobotPrivateKeyCredentials and GoogleRobotMetadataCredentials) are adapted to work with the Jenkins credentials store, but underlying them is a Credential which is used by most Google client libraries. In this example, it's creating a Storage client which can be used for authenticated API requests.

Soon we'll be changing how this is used by incorporating those parts of the google-storage-plugin into the gcp-client module of gcp-plugin-core-java. All of our clients (modified for convenient usage with our plugins) will be created here using the Credential as a HttpRequestInitializer. For examples of that usage you can see our GKE plugin here.