EC2: Make amazon-ssm-agent optional

Move it from VM_EXTRA_PACKAGES in ec2.conf to VM_EXTRA_PACKAGES in
ec2-{base,cloud-init}.conf

Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D46508

(cherry picked from commit 40ff0753abb9c00b5f1e5df9ea00c9fdded55ac4)