Allow option to skip tenant namespace creation

Add --skip-namespace flag to the 'create tenant' command to skip automatic
namespace creation when the namespace already exists.

Signed-off-by: Anshuman Singh <anshumanchauhan9@gmail.com>

cmd/flux/testdata/create_tenant/tenant-with-skip-namespace.yaml

@@ -0,0 +1,27 @@
+---
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  labels:
+    toolkit.fluxcd.io/tenant: dev-team
+  name: dev-team
+  namespace: apps
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: RoleBinding
+metadata:
+  labels:
+    toolkit.fluxcd.io/tenant: dev-team
+  name: dev-team-reconciler
+  namespace: apps
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: cluster-admin
+subjects:
+- apiGroup: rbac.authorization.k8s.io
+  kind: User
+  name: gotk:apps:reconciler
+- kind: ServiceAccount
+  name: dev-team
+  namespace: apps