Provision ~/.yubico/authorized_yubikeys files

Summary:
If a bastion allows an OTP authentication through yubikeys as PAM module,
it needs to read such keys in ~/.yubico/authorized_yubikeys.

This changes provisions such file through the shellusers pillar entry.

Test Plan:

$ cat ~/.yubico/authorized_yubikeys
dereckson:ccccccbjncrt

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1332