Upgrade to terraform 0.12

[itsdalmo]

The only breaking change here should be tags {} becoming tags = {} (terraform 0.12's fault), and the workarounds for instance_policy and user_data being fixed. I think that is a good start, and then we can do more drastic stuff afterwards.

Since terraform 0.12 makes it easier to provide default values/behavior, and expose values, I think it makes sense to do breaking changes later:

• Expose all outputs.
• Expose more variables (as many as possible), and just set default values instead.
• Try launch templates again (see Use aws_launch_template instead of launch configuration? #11).
• Set encryption by default.

But should perhaps include the following changes in this PR:

• Switch to Taskfile instead of Makefile
• Use terratest to run integration tests?

[mikael-lindstrom]

LGTM 🥇