Introduce Key-Value API version in spring.cloud.vault.kv configuration.

See gh-209.

spring-cloud-vault-config/src/main/java/org/springframework/cloud/vault/config/VaultBootstrapPropertySourceConfiguration.java

@@ -142,7 +142,9 @@ public class VaultBootstrapPropertySourceConfiguration implements InitializingBe
 						keyValueBackend, Arrays.asList(applicationContext
 								.getEnvironment().getActiveProfiles()));
 
-				if (keyValueBackend instanceof VaultKeyValueBackendProperties) {
+				if (keyValueBackend instanceof VaultKeyValueBackendProperties
+						&& ((VaultKeyValueBackendProperties) keyValueBackend)
+								.getBackendVersion() == 2) {
 
 					for (String context : contexts) {
 						secretBackendConfigurer.add(KeyValueSecretBackendMetadata.create(

spring-cloud-vault-config/src/main/java/org/springframework/cloud/vault/config/VaultKeyValueBackendProperties.java

@@ -26,7 +26,7 @@ import org.springframework.util.StringUtils;
 import org.springframework.validation.annotation.Validated;
 
 /**
- * Configuration properties for Vault using the generic backend.
+ * Configuration properties for Vault using the key-value backend.
  *
  * @author Mark Paluch
  * @since 2.0
@@ -64,6 +64,15 @@ public class VaultKeyValueBackendProperties implements EnvironmentAware,
 	 */
 	private String applicationName = "application";
 
+	/**
+	 * Key-Value backend version. Currently supported versions are:
+	 * <ul>
+	 * <li>Version 1 (unversioned key-value backend).</li>
+	 * <li>Version 2 (versioned key-value backend).</li>
+	 * </ul>
+	 */
+	private int backendVersion = 2;
+
 	@Override
 	public void setEnvironment(Environment environment) {