Add ability to deploy Kubernetes application to the specified namespa…

…ce (#213)

**What this PR does / why we need it**:

**Which issue(s) this PR fixes**:

Fixes #

**Does this PR introduce a user-facing change?**:
<!--
If no, just write "NONE" in the release-note block below.
-->
```release-note
NONE
```

This PR was merged by Kapetanios.

pkg/app/piped/cloudprovider/kubernetes/kubectl.go

@@ -37,7 +37,7 @@ func NewKubectl(version, path string) *Kubectl {
 	}
 }
 
-func (c *Kubectl) Apply(ctx context.Context, manifest Manifest) (err error) {
+func (c *Kubectl) Apply(ctx context.Context, namespace string, manifest Manifest) (err error) {
 	defer func() {
 		metricsKubectlCalled(c.version, "apply", err == nil)
 	}()
@@ -46,7 +46,14 @@ func (c *Kubectl) Apply(ctx context.Context, manifest Manifest) (err error) {
 	if err != nil {
 		return err
 	}
-	cmd := exec.CommandContext(ctx, c.execPath, "apply", "-f", "-")
+
+	args := make([]string, 0, 5)
+	if namespace != "" {
+		args = append(args, "-n", namespace)
+	}
+	args = append(args, "apply", "-f", "-")
+
+	cmd := exec.CommandContext(ctx, c.execPath, args...)
 	r := bytes.NewReader(data)
 	cmd.Stdin = r
 
@@ -62,10 +69,12 @@ func (c *Kubectl) Delete(ctx context.Context, r ResourceKey) (err error) {
 		metricsKubectlCalled(c.version, "delete", err == nil)
 	}()
 
-	args := []string{"delete", r.Kind, r.Name}
+	args := make([]string, 0, 5)
 	if r.Namespace != "" {
 		args = append(args, "-n", r.Namespace)
 	}
+	args = append(args, "delete", r.Kind, r.Name)
+
 	cmd := exec.CommandContext(ctx, c.execPath, args...)
 	out, err := cmd.CombinedOutput()
 

pkg/app/piped/cloudprovider/kubernetes/kubernetes.go

@@ -165,7 +165,7 @@ func (p *provider) ApplyManifest(ctx context.Context, manifest Manifest) error {
 		return p.initErr
 	}
 
-	return p.kubectl.Apply(ctx, manifest)
+	return p.kubectl.Apply(ctx, p.input.Namespace, manifest)
 }
 
 // Delete deletes the given resource from Kubernetes cluster.

pkg/app/piped/executor/kubernetes/canary.go

@@ -138,6 +138,7 @@ func (e *Executor) ensureCanaryClean(ctx context.Context) model.StageStatus {
 	return model.StageStatus_STAGE_SUCCESS
 }
 
+// TODO: Add namespace for generated resources.
 func (e *Executor) generateCanaryManifests(ctx context.Context, manifests []provider.Manifest, opts config.K8sCanaryRolloutStageOptions) ([]provider.Manifest, error) {
 	// List of default configurations.
 	var (

pkg/config/deployment.go

@@ -318,6 +318,7 @@ type KubernetesDeploymentInput struct {
 	HelmOptions *InputHelmOptions `json:"helmOptions"`
 	HelmVersion string            `json:"helmVersion"`
 
+	// The namespace where manifests will be applied.
 	Namespace string `json:"namespace"`
 	// Automatically reverts all changes from all stages when one of them failed.
 	AutoRollback bool     `json:"autoRollback"`