Avoid writing derived dosh_host in installers

install.sh

@@ -687,9 +687,9 @@ EOF
   if [ "$force_config" -eq 1 ] || [ ! -f "$hosts_config" ]; then
     default_server="${server:-user@example.com}"
     if [ -n "$dosh_host" ]; then
-      host_udp="$dosh_host"
+      host_udp_line="dosh_host = \"$dosh_host\""
     else
-      host_udp="$default_server"
+      host_udp_line="# dosh_host = \"server.example.com\""
     fi
     cat >"$hosts_config" <<EOF
 # Example:
@@ -702,7 +702,7 @@ EOF
 
 [default]
 ssh = "$default_server"
-dosh_host = "$host_udp"
+$host_udp_line
 port = $port
 predict = true
 EOF