EC2: Move network config into a separate function

Having the "base" FreeBSD network configuration (aka. what is used
when not using cloud-init) in ec2.conf will allow us to reuse it in
other AMIs.

Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D46507

(cherry picked from commit f961ddb28d6909d4c67e3e0b6b60498bbcbf64cb)