dashboard: change linux-amd64-longtest machine to n1-highcpu-16

CL 167638 made a change to add more CPU resources to the longtest
builder, with the intention of going from 4 vCPUs, 15 GB RAM to
16 vCPUs, 14.4 GB RAM.

It used n1-highcpu-8 GCE machine type, which actually has 8 vCPUs
and 7.2 GB RAM.¹ Having less RAM than before wasn't the intention.

Fix that by changing n1-highcpu-8 to n1-highcpu-16, which matches
the comment.

¹ https://cloud.google.com/compute/docs/machine-types

Updates golang/go#32831
Updates golang/go#33986
Updates golang/go#25886

Change-Id: I8426867fe33b3bf86576cb13d0d6113cd87f30c1
Reviewed-on: https://go-review.googlesource.com/c/build/+/192679
Reviewed-by: Bryan C. Mills <[email protected]>

Author: dmitshur

dashboard/builders.go

@@ -46,7 +46,7 @@ var Hosts = map[string]*HostConfig{
 	"host-linux-stretch-morecpu": &HostConfig{
 		Notes:           "Debian Stretch, but on n1-highcpu-8",
 		ContainerImage:  "linux-x86-stretch:latest",
-		machineType:     "n1-highcpu-8", // 16 vCPUs, 14.4 GB mem
+		machineType:     "n1-highcpu-16", // 16 vCPUs, 14.4 GB mem
 		buildletURLTmpl: "http://storage.googleapis.com/$BUCKET/buildlet.linux-amd64",
 		env:             []string{"GOROOT_BOOTSTRAP=/go1.4"},
 		SSHUsername:     "root",