nuageinit: Set recommended SSH permissions
    
    As stated in sshd(8), the recommended permissions for ~/.ssh are
    read/write/execute for the user, and not accessible by others; and the
    recommended permissions for ~/.ssh/authorized_keys are read/write for
    the user, and not accessible by others.
    
    (cherry picked from commit 07d17ca189fcf3cc44b7706040b05ca8135c3b85)