refactor golang scripts a bit

Namely ensure the script fails if any step fails, only skip installing
if that particular version is installed, and retry if the download fails

scripts/vagrant-linux-priv-go.sh

@@ -1,16 +1,22 @@
 #!/usr/bin/env bash
 
+set -o errexit
+
 function install_go() {
 	local go_version="1.15.5"
-	local download=
+	local download="https://storage.googleapis.com/golang/go${go_version}.linux-amd64.tar.gz"
 
-	download="https://storage.googleapis.com/golang/go${go_version}.linux-amd64.tar.gz"
-
-	if [ -d /usr/local/go ] ; then
+		if go version 2>&1 | grep -q "${go_version}"; then
 		return
 	fi
 
-	curl -sSL --fail -o /tmp/go.tar.gz ${download}
+	# remove previous older version
+	rm -rf /usr/local/go
+
+	# retry downloading on spurious failure
+	curl -sSL --fail -o /tmp/go.tar.gz \
+		--retry 5 --retry-connrefused \
+		"${download}"
 
 	tar -C /tmp -xf /tmp/go.tar.gz
 	sudo mv /tmp/go /usr/local
@@ -18,7 +24,7 @@ function install_go() {
 }
 
 install_go
-	
+
 # Ensure that the GOPATH tree is owned by vagrant:vagrant
 mkdir -p /opt/gopath
 chown -R vagrant:vagrant /opt/gopath