use java 1.8 on ubuntu 18.04
use java 1.8 on ubuntu 18.04

scylla-jmx doesn't work with latest jdk.

related issue: https://github.com/scylladb/scylla/issues/3858

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-artifact-tests,scylladb/scylla-artifact-tests