Fix YAML parsing for computed namespaces

[lblackstone]

This fixes a bug where the provider was incorrectly concatenating
computed namespaces based on the faulty assumption that they
were always a plain string value rather than an Output.

Fixes #478
Fixes #409

Here's an example of the working output:

$ pulumi up --skip-preview
Updating (yaml-url-step1):

     Type                              Name                                  Status
 +   pulumi:pulumi:Stack               guestbook-yaml-url-step1              created
 +   ├─ kubernetes:yaml:ConfigFile     guestbook                             created
 +   │  ├─ kubernetes:core:Service     test-namespace-enbxii7q/frontend      created
 +   │  ├─ kubernetes:core:Service     test-namespace-enbxii7q/redis-slave   created
 +   │  ├─ kubernetes:core:Service     test-namespace-enbxii7q/redis-master  created
 +   │  ├─ kubernetes:apps:Deployment  test-namespace-enbxii7q/redis-master  created
 +   │  ├─ kubernetes:apps:Deployment  test-namespace-enbxii7q/redis-slave   created
 +   │  └─ kubernetes:apps:Deployment  test-namespace-enbxii7q/frontend      created
 +   └─ kubernetes:core:Namespace      test-namespace                        created

Resources:
    + 9 created

Duration: 14s

[metral]

LGTM