thar-admin: retrieve SSH public keys via IMDSv2

[etungsten]

Issue #, if available: Partially addresses #685

Description of changes:
start_admin_sshd.sh now retrieves SSH public keys via IMDSv2.

Testing done:
Built new thar-admin container image; Launched a Thar instance with it as the admin host-container.

I can SSH to the Thar instance admin container and drop into a root shell with sudo sheltie.

$ ssh ec2-user@SNIP
Last login: Sat Feb  1 00:05:30 2020 from SNIP
Welcome to Thar's Handy Administrator Resources (the admin container)!

This container provides access to the Thar host filesystems (see
/.thar/rootfs) and contains common tools for inspection and troubleshooting.
It is based on Amazon Linux 2, and most things are in the same places you would
find them on an AL2 host.

To permit more intrusive troubleshooting, including actions that mutate the
running state of the Thar host, we provide a tool called "sheltie" (`sudo sheltie`).
When run, this tool drops you into a root shell in the Thar host's root filesystem.
[ec2-user@SNIP ~]$ sudo sheltie
bash-5.0# 

Same test done for both when instances are launched with enforced IMDSv2 and without.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[zmrow]

🏔

[etungsten]

Addresses @jahkeup 's comment.

Tested and admin container still works