fix: replace text example of private key with an image in the jenkins readme

[daniel-cit]

This PR replaces the text example of a RSA private key with an image in the Jenkins readme to prevent.

This example of a RSA private key may trigger a validation when pushing code to source repo to prevent exposure of private keys.

fixes #1014

[daniel-cit]

/gcbrun

[daniel-cit]

@gtsorbo this Pull request is documentation only.
The reason the integration build is triggered is because the png file add is not in the exclusion list of file formats

[apeabody]

@gtsorbo this Pull request is documentation only. The reason the integration build is triggered is because the png file add is not in the exclusion list of file formats

If someone opens a PR for the trigger exclusion I can review it: https://github.com/GoogleCloudPlatform/cloud-foundation-toolkit/blob/master/infra/terraform/test-org/ci-triggers/triggers.tf#L422

[gtsorbo]

@gtsorbo this Pull request is documentation only. The reason the integration build is triggered is because the png file add is not in the exclusion list of file formats

If someone opens a PR for the trigger exclusion I can review it: https://github.com/GoogleCloudPlatform/cloud-foundation-toolkit/blob/master/infra/terraform/test-org/ci-triggers/triggers.tf#L422

GoogleCloudPlatform/cloud-foundation-toolkit#2028 for your consideration @apeabody

[apeabody]

@gtsorbo this Pull request is documentation only. The reason the integration build is triggered is because the png file add is not in the exclusion list of file formats

If someone opens a PR for the trigger exclusion I can review it: https://github.com/GoogleCloudPlatform/cloud-foundation-toolkit/blob/master/infra/terraform/test-org/ci-triggers/triggers.tf#L422

GoogleCloudPlatform/cloud-foundation-toolkit#2028 for your consideration @apeabody

Thanks! I've merged GoogleCloudPlatform/cloud-foundation-toolkit#2027 and applied the update to the triggers.