Adds loadbalanced=false to physical machines and standalone VMs (#266)

We have been adding the metadata file "loadbalanced" with a content/value of
"true" to MIG instances. However, we have not been explicitly flagging
non-loadbalanced machines as loadbalanced=false. This commit makes the
declaration explicity for all machine types, which should make filtering in
BigQuery using this field more intuitive and consitent with other fields.

configs/stage3_ubuntu/opt/mlab/bin/write-metadata.sh

@@ -29,3 +29,7 @@ case "$PREFIX_LEN" in
     ;;
 esac
 echo -n "$MANAGED" > $METADATA_DIR/managed
+
+# Physical machines are not loadbalanced.
+echo -n "false" > $METADATA_DIR/loadbalanced
+

configs/virtual_ubuntu/opt/mlab/bin/write-metadata.sh

@@ -36,6 +36,7 @@ if [[ $is_mig == "200" ]]; then
   external_ip=$(curl "${CURL_FLAGS[@]}" "${METADATA_URL}/network-interfaces/0/forwarded-ips/0")
   external_ipv6=$(curl "${CURL_FLAGS[@]}" "${METADATA_URL}/network-interfaces/0/forwarded-ipv6s/0")
 else
+  echo -n "false" > $METADATA_DIR/loadbalanced
   external_ip=$(
     curl "${CURL_FLAGS[@]}" "${METADATA_URL}/network-interfaces/0/access-configs/0/external-ip"
   )