Rename and document nsk-test SSH key in authorized_keys

[dnesting]

ami-020bc76a seems to contain an SSH key in .ssh/authorized_keys labeled "nsk-test". At first glance this seems super sketchy and I immediately commented it out. If it's here for legitimate purposes (someone suggested it was for a security scan), consider doing some or all of:

1. Rename the key to something that identifies its purpose
2. Add a brief comment to the authorized_keys file explaining why it's there and why one should leave it or comment it out.
3. Document somewhere how the private key is protected because this seems like a very sensitive key that deserves some protection above and beyond a typical user key