Use @*@ delimiters in Maven resource filtering

It *is* very useful to have filtering on by default, so that is now
the case (in spring-boot-starter-parent). Users can filter resources
by default by adding @*@ placeholders (so as not to clash with Spring
${} placeholders).

Fixes gh-1199

spring-boot-samples/spring-boot-sample-actuator/src/main/resources/application.properties

@@ -18,4 +18,7 @@ shell.auth: spring
 #shell.auth.key.path: ${user.home}/test/id_rsa.pub.pem
 #shell.auth: simple
 spring.jmx.enabled: true
-
+info.group: @project.groupId@
+info.artifact: @project.artifactId@
+info.name: @project.name@
+info.version: @project.version@