Add Ropsten to DNS Discovery (#1629)

DNS Discovery had options for mainnet, rinkeby, and goerli, but Ropsten
was left out.

Signed-off-by: Danno Ferrin <danno.ferrin@gmail.com>

besu/src/main/java/org/hyperledger/besu/cli/config/EthNetworkConfig.java

@@ -25,6 +25,7 @@ import static org.hyperledger.besu.ethereum.p2p.config.DiscoveryConfiguration.MO
 import static org.hyperledger.besu.ethereum.p2p.config.DiscoveryConfiguration.RINKEBY_BOOTSTRAP_NODES;
 import static org.hyperledger.besu.ethereum.p2p.config.DiscoveryConfiguration.RINKEBY_DISCOVERY_URL;
 import static org.hyperledger.besu.ethereum.p2p.config.DiscoveryConfiguration.ROPSTEN_BOOTSTRAP_NODES;
+import static org.hyperledger.besu.ethereum.p2p.config.DiscoveryConfiguration.ROPSTEN_DISCOVERY_URL;
 import static org.hyperledger.besu.ethereum.p2p.config.DiscoveryConfiguration.YOLO_V2_BOOTSTRAP_NODES;
 
 import org.hyperledger.besu.ethereum.p2p.peers.EnodeURL;
@@ -131,7 +132,10 @@ public class EthNetworkConfig {
     switch (networkName) {
       case ROPSTEN:
         return new EthNetworkConfig(
-            jsonConfig(ROPSTEN_GENESIS), ROPSTEN_NETWORK_ID, ROPSTEN_BOOTSTRAP_NODES, null);
+            jsonConfig(ROPSTEN_GENESIS),
+            ROPSTEN_NETWORK_ID,
+            ROPSTEN_BOOTSTRAP_NODES,
+            ROPSTEN_DISCOVERY_URL);
       case RINKEBY:
         return new EthNetworkConfig(
             jsonConfig(RINKEBY_GENESIS),

ethereum/p2p/src/main/java/org/hyperledger/besu/ethereum/p2p/config/DiscoveryConfiguration.java

@@ -33,6 +33,8 @@ public class DiscoveryConfiguration {
       "enrtree://AKA3AM6LPBYEUDMVNU3BSVQJ5AD45Y7YPOHJLEF6W26QOE4VTUDPE@all.mainnet.ethdisco.net";
   public static final String RINKEBY_DISCOVERY_URL =
       "enrtree://AKA3AM6LPBYEUDMVNU3BSVQJ5AD45Y7YPOHJLEF6W26QOE4VTUDPE@all.rinkeby.ethdisco.net";
+  public static final String ROPSTEN_DISCOVERY_URL =
+      "enrtree://AKA3AM6LPBYEUDMVNU3BSVQJ5AD45Y7YPOHJLEF6W26QOE4VTUDPE@all.ropsten.ethdisco.net";
 
   public static final List<EnodeURL> MAINNET_BOOTSTRAP_NODES =
       Collections.unmodifiableList(