ubuntu 14.04: enable openjdk repo for installing scylla 1.7 and later
ubuntu 14.04: enable openjdk repo for installing scylla 1.7 and later

In scylla 1.7, scylla-jmx and scylla-tools depend on openjdk packages.
This patch enables openjdk repo for install scylla 1.7 and later.

https://github.com/scylladb/scylla/issues/2091

v2: compare with `1.7~rc0`, get exact scylla version
v3: install and switch to openjdk8

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

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