sshkeys: do not fail if no keys are deployed

[zyclonite]

Bug

Operating System Version

30.20191014.1 (CoreOS preview)
(happened in previous versions as well)

Afterburn Version

4.1.3

Environment

aws

Expected Behavior

no failing systemd unit after boot

Actual Behavior

● afterburn-sshkeys@core.service - Afterburn (SSH Keys)
   Loaded: loaded (/usr/lib/systemd/system/afterburn-sshkeys@.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Thu 2019-11-28 10:25:13 UTC; 8min ago
  Process: 934 ExecStart=/usr/bin/afterburn ${AFTERBURN_OPT_PROVIDER} --ssh-keys=core (code=exited, status=1/FAILURE)
 Main PID: 934 (code=exited, status=1/FAILURE)

Nov 28 10:25:12 ip-10-100-20-105.eu-west-1.compute.internal systemd[1]: Starting Afterburn (SSH Keys)...
Nov 28 10:25:13 ip-10-100-20-105.eu-west-1.compute.internal afterburn[934]: Nov 28 10:25:13.314 INFO Fetching http://169.254.169.254/2009-04-04/meta-data/public-keys: Attempt #1
Nov 28 10:25:13 ip-10-100-20-105.eu-west-1.compute.internal afterburn[934]: Nov 28 10:25:13.327 INFO Fetch failed with 404: resource not found
Nov 28 10:25:13 ip-10-100-20-105.eu-west-1.compute.internal afterburn[934]: Error: writing ssh keys
Nov 28 10:25:13 ip-10-100-20-105.eu-west-1.compute.internal afterburn[934]: Caused by: failed to open "/var/home/core/.ssh/authorized_keys.d" for syncing
Nov 28 10:25:13 ip-10-100-20-105.eu-west-1.compute.internal afterburn[934]: Caused by: No such file or directory (os error 2)
Nov 28 10:25:13 ip-10-100-20-105.eu-west-1.compute.internal systemd[1]: afterburn-sshkeys@core.service: Main process exited, code=exited, status=1/FAILURE
Nov 28 10:25:13 ip-10-100-20-105.eu-west-1.compute.internal systemd[1]: afterburn-sshkeys@core.service: Failed with result 'exit-code'.
Nov 28 10:25:13 ip-10-100-20-105.eu-west-1.compute.internal systemd[1]: Failed to start Afterburn (SSH Keys).

Reproduction Steps

1. start new ec2 instance without providing ssh keys
2. ssh into instance and check for failed systemd units

Other Information

[lucab]

Thanks for the report.
This is indeed a buggy interaction between the file writing logic and the set of SSH keys being empty.

[lucab]

PR at #318.