tomcat-native: find java_home and tomcat-native

- Use `/usr/libexec/java_home` to find `$JAVA_HOME` instead of hard-coding a path. This makes the formula more resistant to breakage should Apple or Oracle decide to move things around or perhaps even if people use exotic or esoteric JVMs.
- Update the caveat message to suggest a less invasive way to help tomcat discover tomcat-native. No need to leave the Cellar for this one.