Add support for multiple credhubs

[MatthiasWinzeler]

Currently, Diego supports interpolating credentials only from a single Credhub (the runtime Credhub as it's called in the docs). The URL to this Credhub is passed from CC to Diego in VCAP_PLATFORM_OPTIONS.

In our CF, we'd like to offer multiple Credhubs to our users.
More specifically, each user can get its own Credhub (as a service instance from the marketplace).

To tightly integrate this with CF, we'd like to add Diego support so that Diego automatically interpolates credentials from the user's credhub instance. However, this is currently limited by Diego's ability to speak to one platform credhub only.

I have two ideas how to add support for multiple credhubs. Both basically add a credhub url to the service binding, which then Diego would respect:

• Extend credhub-ref so that it can contain an optional full URL that points to Credhub, for example "credhub-ref": "https://<url-to-credhub-xyz>/<something>/credhub/171d5272-a269-42bc-9913-5a288384a746/credentials"
• Add a new credhub-url that is passed in the binding next to credhub-ref

What do you think about this?

We're happy to provide a PR if this is also in your interest; it would be rather small changes to both
https://github.com/cloudfoundry/dockerapplifecycle and https://github.com/cloudfoundry/buildpackapplifecycle

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/159161412

The labels on this github issue will be updated when the story is started.

[heyjcollins]

As this item has been open for quite a long time, and given current priorities and resource availability we're not going to be in a position to work on this so I'm going to go ahead and close it.