EC2: Add experimental cloud-init images
Known issues:
- The ec2-user user is created with a homedir of /usr/home/ec2-user
instead of /home/ec2-user; this appears to be a bug in cloud-init's
FreeBSD support.
- Cloud-init configures IPv4 networking but not IPv6 networking.
releng/14.0 candidate.
Discussed with: gjb
Reviewed by: imp
MFC after: 5 days
Relnotes: yes
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41793