fix(kubernetes): Deploy to a numeric k8s namespace fails (#1597)

Allows deployment to a Kubernetes namespace that is numeric.

KubernetesV2Utils is mapping the deployment environment values to Map<String, Object>, and number-ish values are interpreted as numeric types by YAML.
This caused a numeric namespace value to get interpreted as a Long, and later hard-casts to String in KubernetesV2Executor.exists() threw a ClassCastException

Example failing config:

```
kubernetes:
- accounts:
  - name: foobar
    ...
    namespaces:
    - 123412341234
...
deploymentEnvironment:
  location: 123412341234
```

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

halyard-deploy/src/main/resources/kubernetes/manifests/deployment.yml

@@ -2,7 +2,7 @@ apiVersion: apps/v1 # As of Kubernetes 1.16, apps/v1beta2 has been deprecated
 kind: Deployment
 metadata:
   name: spin-{{ name }}
-  namespace: {{ namespace }}
+  namespace: "{{ namespace }}"
   annotations:
     moniker.spinnaker.io/application: '"spin"'
     moniker.spinnaker.io/cluster: '"{{ name }}"'

halyard-deploy/src/main/resources/kubernetes/manifests/namespace.yml

@@ -1,6 +1,6 @@
 kind: Namespace
 apiVersion: v1
 metadata:
-  name: {{ name }}
+  name: "{{ name }}"
   labels:
     app: spin

halyard-deploy/src/main/resources/kubernetes/manifests/service.yml

@@ -2,7 +2,7 @@ kind: Service
 apiVersion: v1
 metadata:
   name: spin-{{ name }}
-  namespace: {{ namespace }}
+  namespace: "{{ namespace }}"
   labels:
     app: spin
     cluster: spin-{{ name }}