Enable multiple instances of YAML manifests per stack

[lblackstone]

Fixes #536

With this fix, the following snippet produces this output:

new k8s.yaml.ConfigFile("guestbook",
    {
        file: "https://raw.githubusercontent.com/pulumi/pulumi-kubernetes/master/tests/examples/yaml-guestbook/yaml/guestbook.yaml",
        resourcePrefix: "prod",
    },
    {providers: {kubernetes: k8sProvider}}
);
new k8s.yaml.ConfigFile("guestbook",
    {
        file: "https://raw.githubusercontent.com/pulumi/pulumi-kubernetes/master/tests/examples/yaml-guestbook/yaml/guestbook.yaml",
        resourcePrefix: "local",
    }
);

     Type                                                        Name                         Plan
 +   pulumi:pulumi:Stack                                         gke-ingress-gke-ingress-dev  create
 +   ├─ kubernetes:yaml:ConfigFile                               prod-guestbook               create
 +   │  ├─ kubernetes:core:Service                               prod-frontend                create
 +   │  ├─ kubernetes:core:Service                               prod-redis-slave             create
 +   │  ├─ kubernetes:core:Service                               prod-redis-master            create
 +   │  ├─ kubernetes:apps:Deployment                            prod-redis-master            create
 +   │  ├─ kubernetes:apps:Deployment                            prod-frontend                create
 +   │  └─ kubernetes:apps:Deployment                            prod-redis-slave             create
 +   ├─ kubernetes:yaml:ConfigFile                               local-guestbook              create
 +   │  ├─ kubernetes:core:Service                               local-redis-slave            create
 +   │  ├─ kubernetes:core:Service                               local-frontend               create
 +   │  ├─ kubernetes:core:Service                               local-redis-master           create
 +   │  ├─ kubernetes:apps:Deployment                            local-redis-slave            create
 +   │  ├─ kubernetes:apps:Deployment                            local-redis-master           create
 +   │  └─ kubernetes:apps:Deployment                            local-frontend               create
 +   ├─ gcp:container:Cluster                                    gke-cluster                  create
 +   ├─ pulumi:providers:kubernetes                              gkeK8s                       create
 +   └─ kubernetes:rbac.authorization.k8s.io:ClusterRoleBinding  levi-cluster-admin           create

[lblackstone]

CI failing due to #591 #592. New test coverage passed.
Edit: Fixed CI.

[metral]

LGTM