Use the system certificate store if no certificates are specified. (#1261)

* Use the system certificate store if no certificates are specified.

* Don't use ServerCertificateCustomValidationCallback when no CA is set

tests/KubernetesClient.Tests/assets/kubeconfig.tls-no-skip.yml

@@ -0,0 +1,22 @@
+# Sample file based on https://kubernetes.io/docs/tasks/access-application-cluster/authenticate-across-clusters-kubeconfig/
+# WARNING: File includes minor fixes
+---
+current-context: federal-context
+apiVersion: v1
+clusters:
+- cluster:
+    insecure-skip-tls-verify: false
+    server: https://horse.org:443
+  name: horse-cluster
+contexts:
+- context:
+    cluster: horse-cluster
+    namespace: chisel-ns
+    user: green-user
+  name: federal-context
+kind: Config
+users:
+- name: green-user
+  user:
+    password: secret
+    username: admin