Store and display credentials for 3rd party services

A use case we have is to be a single point for an account or cloud owner to make credentials to external services (hosted outside of the cloud) to developers. For instance, if an account owner signs up for a Sendgrid account, be able to make that available in one place to users under that account. This later also allows extensions where Murano can use external APIs with preconfigured credentials (an example would be creating a mongodb account hosted by an external vendor).

It would make sense for this to use Barbican, which is designed to store encrypted credentials, and as such, this might be something for after v1. Since Barbican currently doesn't allow the user to specify resource names, we would either need to add that ability to Barbican, or create the Barbican resource and store the result in Murano (separate from Environments, since they should be shared).

Requires https://blueprints.launchpad.net/murano/+spec/packages-without-classes