Add add_svcs, fix nvme, add eu-north-1

Ability to enable additional services on the AMI's runlevels. I'm using this to add haveged to the boot runlevel to boost the amount of initial entropy on smaller instance types, so sshd can start in under 6s instead of over 2m. (I rc-update del haveged boot after sshd is up.) Example variables.yaml snippet:

add_svcs:
  boot:
    - haveged
  default:
    - docker
    - nginx

There was also a race condition I encountered with the NVMe-to-EBS device mapping. Occasionally, the EBS device (/dev/sdX, /dev/xvdX) wasn't available the first time we tried to get it with nvme id-ctrl. Subsequent attempts succeed, however.

This PR also adds the new eu-north-1 (Stockholm) region.