Make default context behaves like a real context:

- when using "--context default" parameter
- when printing the list of contexts
- when exporting the default context to a tarball

Signed-off-by: Jean-Christophe Sirot <jean-christophe.sirot@docker.com>
(+1 squashed commit)
Squashed commits:
[20670495] Fix CLI initialization for the `docker stack deploy --help` command and ensure that the dockerCli.CurrentContext() always returns a non empty context name (default as a fallback)
Remove now obsolete code handling empty string context name
Minor code cleanup

Signed-off-by: Jean-Christophe Sirot <jean-christophe.sirot@docker.com>

cli/command/testdata/test-kubeconfig

@@ -0,0 +1,20 @@
+apiVersion: v1
+clusters:
+- cluster:
+    certificate-authority-data: dGhlLWNh
+    server: https://someserver
+  name: test-cluster
+contexts:
+- context:
+    cluster: test-cluster
+    user: test-user
+    namespace: zoinx
+  name: test
+current-context: test
+kind: Config
+preferences: {}
+users:
+- name: test-user
+  user:
+    client-certificate-data: dGhlLWNlcnQ=
+    client-key-data: dGhlLWtleQ==