EC2: Respect ${VMFS}

Prior to this commit, ${VMFS} controlled the filesystem used for
building EC2 images, but the AMIs were recorded with SSM Parameter
paths which indicated that they were UFS.

This commit (a) uses ${VMFS} in the SSM parameter path instead of
a hard-coded "ufs", and (b) adds the filesystem to the AMI name.

Reviewed by: gjb
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41342