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)