Allow system property override to be switched off

The default behaviour is the same as before, so a config client
adds the remote property sources "first" (i.e. ahead of system
properties). If the user sets up a  remote config repo with
spring.cloud.config.overrideSystemProperties=false they can
change this behaviour and insert the new property source after
systemEnvironment (i.e. before local config files but after the
other local sources). Of course using an `application.yml` on
the server you can change the default  for all applications.

There is also a new feature in the config server where the operator
can add a map of override properties in
spring.cloud.config.server.overrides.* and have them added with
highest priority in the Environment returned from the server.
Using that the operator can prevent config repositories from
changing the override behaviour by setting
spring.cloud.config.allowOverride=false.

Fixes gh-57

spring-cloud-config-server/src/test/java/org/springframework/cloud/config/server/ApplicationTests.java

@@ -1,5 +1,6 @@
 package org.springframework.cloud.config.server;
 
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 
 import java.io.IOException;
@@ -35,6 +36,8 @@ public class ApplicationTests {
 	public void contextLoads() {
 		Environment environment = new TestRestTemplate().getForObject("http://localhost:" + port + "/foo/development/", Environment.class);
 		assertFalse(environment.getPropertySources().isEmpty());
+		assertEquals("overrides", environment.getPropertySources().get(0).getName());
+		assertEquals("{spring.cloud.config.enabled=true}", environment.getPropertySources().get(0).getSource().toString());
 	}
 
 }

spring-cloud-config-server/src/test/java/org/springframework/cloud/config/server/EnvironmentControllerTests.java

@@ -17,6 +17,7 @@ package org.springframework.cloud.config.server;
 
 import static org.junit.Assert.assertEquals;
 
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -38,7 +39,7 @@ import org.springframework.test.web.servlet.setup.MockMvcBuilders;
  *
  */
 public class EnvironmentControllerTests {
-	
+
 	@Rule
 	public ExpectedException expected = ExpectedException.none();
 
@@ -120,9 +121,22 @@ public class EnvironmentControllerTests {
 
 	@Test
 	public void mappingForLabelledYamlWithHyphen() throws Exception {
-		Mockito.when(repository.findOne("foo", "bar-spam", "other")).thenReturn(environment);
+		Mockito.when(repository.findOne("foo", "bar-spam", "other")).thenReturn(
+				environment);
 		MockMvc mvc = MockMvcBuilders.standaloneSetup(controller).build();
 		mvc.perform(MockMvcRequestBuilders.get("/other/foo-bar-spam.yml")).andExpect(
 				MockMvcResultMatchers.status().isBadRequest());
 	}
+
+	@Test
+	public void allowOverrideFalse() throws Exception {
+		controller.setOverrides(Collections.singletonMap("foo", "bar"));
+		Map<String, Object> map = new HashMap<String, Object>();
+		map.put("a.b.c", "d");
+		environment.add(new PropertySource("one", map));
+		Mockito.when(repository.findOne("foo", "bar", "master")).thenReturn(environment);
+		assertEquals("{foo=bar}", controller.master("foo", "bar").getPropertySources()
+				.get(0).getSource().toString());
+	}
+
 }

spring-cloud-config-server/src/test/resources/configserver-test.yml

@@ -4,3 +4,8 @@ spring:
       server:
         git:
           basedir: target/config
+        overrides:
+          spring:
+            cloud:
+              config:
+                enabled: true