Dependency graph support

Some services logically depend on other services.

For example, if you provision a new service with a stable hostname or IP somewhere (an [AWS Elastic Load Balancer](https://aws.amazon.com/elasticloadbalancing/) gives you a hostname), you might want to also set up a DNS A record or CNAME.

This issue is a proposal for steward-framework to be aware of these logical dependencies by having a dependency graph stored somewhere, and traversing that dependency graph to fulfill a service request _and_ its logical dependencies when a `Instance` is created for the original service.

Moved from https://github.com/deis/steward/issues/11