Detect `user_data` failure

[lawliet89]

We run pretty elaborate scripts in the user_data portions of the EC2 instances.

We need some way to detect if these scripts have failed.

Probabilities:

• https://www.uvd.co.uk/blog/create-health-check-aws-user-data-script
• https://stackoverflow.com/questions/11245356/how-to-check-user-data-status-while-launching-the-instance-in-aws

Idea:

• Define the existence of a user_data completion marker file as a service health check in Consul
• Forward logs via td-agent from User_data

TODOs:

• Consul Server
• Consul Agents (#131)
• Forward user_data logs via td-agent