Allow auto-configured caches to be overridden

This commit updates the JSR 107 cache auto-configuration to only create
a cache if it does not already exist. It also puts a specific order for
the auto-configured customizer so that another instance can run prior to
it reliably.

Closes gh-1157

initializr-web/pom.xml

@@ -98,6 +98,11 @@
 			<artifactId>spring-restdocs-mockmvc</artifactId>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.ehcache</groupId>
+			<artifactId>ehcache</artifactId>
+			<scope>test</scope>
+		</dependency>
 		<dependency>
 			<groupId>com.vaadin.external.google</groupId>
 			<artifactId>android-json</artifactId>