Add logic for "families" for GCE images.

This allows for GCE consumers to easily detect the latest major
version of FreeBSD when using the gcloud command line utility.

To ensure snapshot builds do not conflict with release-style
builds (ALPHA, BETA, RC, RELEASE), the '-snap' suffix is appended
to the GCE image family name.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation