Use JDK 9-slim cacerts for OpenJDK 9
Use JDK 9-slim cacerts for OpenJDK 9

OpenJDK 9 images have empty cacert files which was reported in
https://github.com/docker-library/openjdk/issues/145 and http://bugs.java.com/view_bug.do?bug_id=8189357 .

Here we use a cacert cache from `openjdk:9-slim` image that seems to be
populated correctly.

circleci/circleci-images,circleci/circleci-images