EC2: Split off reusable configuration bits

Split ec2-base.conf into ec2-base.conf and a reusable ec2.conf,
similar to how Vagrant flavours share a common vagrant.conf.

releng/14.0 candidate.

Discussed with: gjb
MFC after: 5 days
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41792