[ec2/azure] discovery plugins must declare their UnicastHostsProvider

Closes #13492
(cherry picked from commit 1e233b4)
(cherry picked from commit 0fe4131)

plugins/cloud-azure/src/main/java/org/elasticsearch/plugin/cloud/azure/CloudAzurePlugin.java

@@ -26,6 +26,7 @@ import org.elasticsearch.common.logging.Loggers;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.discovery.DiscoveryModule;
 import org.elasticsearch.discovery.azure.AzureDiscovery;
+import org.elasticsearch.discovery.azure.AzureUnicastHostsProvider;
 import org.elasticsearch.index.snapshots.blobstore.BlobStoreIndexShardRepository;
 import org.elasticsearch.index.store.IndexStoreModule;
 import org.elasticsearch.index.store.smbmmapfs.SmbMmapFsIndexStore;
@@ -80,6 +81,7 @@ public class CloudAzurePlugin extends Plugin {
 
     public void onModule(DiscoveryModule discoveryModule) {
         discoveryModule.addDiscoveryType("azure", AzureDiscovery.class);
+        discoveryModule.addUnicastHostProvider(AzureUnicastHostsProvider.class);
     }
 
     public void onModule(IndexStoreModule storeModule) {

plugins/cloud-gce/src/main/java/org/elasticsearch/plugin/cloud/gce/CloudGcePlugin.java

@@ -25,6 +25,7 @@ import org.elasticsearch.common.inject.Module;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.discovery.DiscoveryModule;
 import org.elasticsearch.discovery.gce.GceDiscovery;
+import org.elasticsearch.discovery.gce.GceUnicastHostsProvider;
 import org.elasticsearch.plugins.Plugin;
 
 import java.util.ArrayList;
@@ -72,6 +73,7 @@ public class CloudGcePlugin extends Plugin {
 
     public void onModule(DiscoveryModule discoveryModule) {
         discoveryModule.addDiscoveryType("gce", GceDiscovery.class);
+        discoveryModule.addUnicastHostProvider(GceUnicastHostsProvider.class);
     }
 
 }

plugins/discovery-ec2/src/main/java/org/elasticsearch/plugin/discovery/ec2/Ec2DiscoveryPlugin.java

@@ -24,6 +24,7 @@ import org.elasticsearch.cloud.aws.Ec2Module;
 import org.elasticsearch.common.component.LifecycleComponent;
 import org.elasticsearch.common.inject.Module;
 import org.elasticsearch.discovery.DiscoveryModule;
+import org.elasticsearch.discovery.ec2.AwsEc2UnicastHostsProvider;
 import org.elasticsearch.discovery.ec2.Ec2Discovery;
 import org.elasticsearch.plugins.Plugin;
 
@@ -61,5 +62,6 @@ public class Ec2DiscoveryPlugin extends Plugin {
 
     public void onModule(DiscoveryModule discoveryModule) {
         discoveryModule.addDiscoveryType("ec2", Ec2Discovery.class);
+        discoveryModule.addUnicastHostProvider(AwsEc2UnicastHostsProvider.class);
     }
 }