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

release/tools/gce.conf

@@ -6,10 +6,14 @@
 # The default of 3GB is too small for GCE, so override the size here.
 export VMSIZE=20G
 
+# The sysutils/py-google-compute-engine port needs the lang/python
+# symbolic link to point to lang/python2.
+export DEFAULT_VERSIONS="python=2.7"
+
 # Set to a list of packages to install.
 export VM_EXTRA_PACKAGES="firstboot-freebsd-update firstboot-pkgs \
 	google-cloud-sdk panicmail sudo sysutils/py-google-compute-engine \
-	lang/python lang/python2 lang/python3"
+	lang/python lang/python2"
 
 # Set to a list of third-party software to enable in rc.conf(5).
 export VM_RC_LIST="ntpd sshd growfs \

release/tools/vmimage.subr

@@ -72,6 +72,10 @@ cleanup() {
 		mdconfig -d -u ${mddev}
 	fi
 
+	# Avoid inheriting the DEFAULT_VERSIONS for lang/python from
+	# the gce.conf.
+	unset DEFAULT_VERSIONS
+
 	return 0
 }