Allow environment overrides to be escaped for placeholders

E.g. spring.cloud.config.server.overrides.foo=$\{bar} will evaluate
to foo=${bar} in the client's Environment (allowing local replacement
of the placeholder instead of on the configserver).

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

@@ -234,4 +234,15 @@ public class EnvironmentControllerTests {
 				.get(0).getSource().toString());
 	}
 
+	@Test
+	public void overrideWithEscapedPlaceholders() throws Exception {
+		controller.setOverrides(Collections.singletonMap("foo", "$\\{bar}"));
+		Map<String, Object> map = new HashMap<String, Object>();
+		map.put("bar", "foo");
+		environment.add(new PropertySource("one", map));
+		Mockito.when(repository.findOne("foo", "bar", "master")).thenReturn(environment);
+		assertEquals("{foo=${bar}}", controller.defaultLabel("foo", "bar").getPropertySources()
+				.get(0).getSource().toString());
+	}
+
 }