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/ca.pem

@@ -0,0 +1,18 @@
+-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

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==