Add dsl spec relationships

Already mentioned above in 2 places.
It is worth stressing the fact that having the interfaces defined under `source_interfaces` and `target_interfaces` does not necessarily mean they will be executed. What I mean by that is that operations defined in `cloudify.interfaces.relationship_lifecycle` will be executed when running `install/uninstall` workflows. But if I want, I can add a custom relationship interface and write a custom workflow that will execute operations from this interface