Support priority ordering for VaultPropertySourceLocator.

We now support setting a priority for VaultPropertySourceLocator by configuring `spring.cloud.vault.config.order`. Lower values increase priority, higher values decrease priority. Applying a priority can be used to control VaultPropertySourceLocator precedence over other property sources.

Original pull request: gh-36.

README.adoc

@@ -180,6 +180,8 @@ spring.cloud.vault:
     scheme: https
     connection-timeout: 5000
     read-timeout: 15000
+    config:
+        order: -10
 ----
 
 * `host` sets the hostname of the Vault host. The host name will be used

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

@@ -29,6 +29,7 @@ import org.springframework.core.env.CompositePropertySource;
 import org.springframework.core.env.ConfigurableEnvironment;
 import org.springframework.core.env.Environment;
 import org.springframework.core.env.PropertySource;
+import org.springframework.core.PriorityOrdered;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
@@ -40,7 +41,7 @@ import static org.springframework.cloud.vault.config.SecureBackendAccessors.*;
  * @author Spencer Gibb
  * @author Mark Paluch
  */
-class VaultPropertySourceLocator implements PropertySourceLocator {
+class VaultPropertySourceLocator implements PropertySourceLocator, PriorityOrdered {
 
 	private final VaultConfigOperations operations;
 	private final VaultProperties properties;
@@ -85,6 +86,11 @@ class VaultPropertySourceLocator implements PropertySourceLocator {
 		return null;
 	}
 
+	@Override
+	public int getOrder() {
+		return properties.getConfig().getOrder();
+	}
+
 	private List<String> buildContexts(ConfigurableEnvironment env) {
 
 		String appName = env.getProperty("spring.application.name");

spring-cloud-vault-core/src/main/java/org/springframework/cloud/vault/VaultProperties.java

@@ -78,6 +78,8 @@ public class VaultProperties {
 
 	private Ssl ssl = new Ssl();
 
+	private Config config = new Config();
+
 	/**
 	 * Application name for AppId authentication.
 	 */
@@ -177,6 +179,15 @@ public class VaultProperties {
 		private String certAuthPath = "cert";
 	}
 
+	@Data
+	public static class Config {
+		/**
+		 * Used to force a PropertySourceLocator ordering.
+		 * This is useful to use Vault as an override on consul properties;
+		 */
+		private int order = 0;
+	}
+
 	public enum AuthenticationMethod {
 		TOKEN, APPID, AWS_EC2, CERT
 	}