MFC r333374:
Use vYYYYMMDD in the timestamp suffix for Google Compute Engine
snapshot images for consistency with other OSes.

Sponsored by: The FreeBSD Foundation

MFC r333473:
Add a special GCE_LICENSE variable to Makefile.gce, which when set,
will include license metadata in the resultant GCE image.

GCE_LICENSE is unset by default, as it primarily pertains to images
produced by the FreeBSD Project, but for downstream FreeBSD consumers,
it can be set in the make(1) environment in the format of:

--licenses="projects/PROJECT_ID/global/licenses/LICENSE_NAME"

The "license" is not a license, per se, but required metadata that
is required by the GCE marketplace. For the FreeBSD Project, the
license name is simply 'freebsd', with the description of 'FreeBSD'.

Approved by: re (marius)
Sponsored by: The FreeBSD Foundation

MFC r331806:
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.

Sponsored by: The FreeBSD Foundation

MFC r303782:
Fix GCE image publication. The gcutil utility is deprecated in favor
of gcloud.

Approved by: re (delphij, kib)
Sponsored by: The FreeBSD Foundation