More verbose message on preserving plugin config

This commit expands on the message printed when config files are
preserved when removing a plugin to give the user an indication of the
reason the config files are preserved.

core/src/main/java/org/elasticsearch/plugins/RemovePluginCommand.java

@@ -93,7 +93,9 @@ final class RemovePluginCommand extends SettingCommand {
         // a message so the user knows in case they want to remove manually
         final Path pluginConfigDir = env.configFile().resolve(pluginName);
         if (Files.exists(pluginConfigDir)) {
-            terminal.println("-> Preserving plugin config files: " + pluginConfigDir);
+            terminal.println(
+                    "-> Preserving plugin config files [" + pluginConfigDir + "] in case of upgrade, delete manually if not needed");
         }
     }
+
 }

qa/evil-tests/src/test/java/org/elasticsearch/plugins/RemovePluginCommandTests.java

@@ -119,7 +119,7 @@ public class RemovePluginCommandTests extends ESTestCase {
         Files.createFile(configDir.resolve("fake.yml"));
         final MockTerminal terminal = removePlugin("fake", home);
         assertTrue(Files.exists(env.configFile().resolve("fake")));
-        assertThat(terminal.getOutput(), containsString("-> Preserving plugin config files: " + configDir));
+        assertThat(terminal.getOutput(), containsString(expectedConfigDirPreservedMessage(configDir)));
         assertRemoveCleaned(env);
     }
 
@@ -127,7 +127,11 @@ public class RemovePluginCommandTests extends ESTestCase {
         Files.createDirectories(env.pluginsFile().resolve("fake"));
         final Path configDir = env.configFile().resolve("fake");
         final MockTerminal terminal = removePlugin("fake", home);
-        assertThat(terminal.getOutput(), not(containsString("-> Preserving plugin config files: " + configDir)));
+        assertThat(terminal.getOutput(), not(containsString(expectedConfigDirPreservedMessage(configDir))));
+    }
+
+    private String expectedConfigDirPreservedMessage(final Path configDir) {
+        return "-> Preserving plugin config files [" + configDir + "] in case of upgrade, delete manually if not needed";
     }
 
 }