upload-git-mirror: no_log around key writing

Add no_log here as a fallback against ever leaving the output in the
logs.

Change-Id: Ia3d518d915705b40e0e12e25e0a0787a8cf614d7

roles/upload-git-mirror/tasks/main.yaml

@@ -10,6 +10,7 @@
         content: "{{ git_mirror_credentials.ssh_key }}"
         dest: "{{ ssh_private_key_tmp.path }}"
         mode: 0600
+      no_log: true
 
     # If the markers in an id_rsa don't end with a newline some
     # versions of ssh won't read the key.