[sonic-slave]: install openjdk-8 for jenkins slave (#3037)

Signed-off-by: Guohan Lu <gulv@microsoft.com>

sonic-slave/Dockerfile

@@ -321,3 +321,8 @@ RUN add-apt-repository \
 RUN apt-get update
 RUN apt-get install -y docker-ce=17.03.2~ce-0~debian-jessie
 RUN echo "DOCKER_OPTS=\"--experimental --storage-driver=vfs\"" >> /etc/default/docker
+
+# For jenkins slave
+RUN echo "deb http://archive.debian.org/debian jessie-backports main" >> /etc/apt/sources.list
+RUN apt-get -o Acquire::Check-Valid-Until=false update
+RUN apt-get -y -o Acquire::Check-Valid-Until=false install ca-certificates-java=20161107~bpo8+1 openjdk-8-jdk