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.

Approved by: re (gjb)
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41792

(cherry picked from commit fada6e2389fb62ff621a98fab7319e426da58b0b)
(cherry picked from commit f4576ea0d5bcc26d8ffdf033cab36c651a5f0885)