Fix GCE virtual machine startup.

The ports/head branch recently switched to python3 as the default,
which breaks the sysutils/py-google-compute-engine startup scripts,
as lang/python installs lang/python3{,.x} where lang/python2{,.x}
are needed.

Set DEFAULT_VERSIONS in release/tools/gce.conf to python=2.7, and
remove the lang/python3 inclusion in VM_EXTRA_PACKAGES.

Additionally, unset DEFAULT_VERSIONS in release/tools/vmimage.subr
to prevent persistence of DEFAULT_VERSIONS=python=2.7 in subsequent
VM/cloud image builds.

Note: at present, this affects only 13-CURRENT and 12-STABLE, as
the stable/11 branch had already switched to using the 2019Q2 branch
at the start of the 11.3-RELEASE cycle, so this does not immediately
affect 11.3-BETA, hence the 1-week merge timeout. This had been
manually tested on 13-CURRENT.

Reported by: ler (privately)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation