gce: set keep-alive tag for long time job
gce: set keep-alive tag for long time job

We try to stop instances without this flag to save
resource, so it's necessary for long time job.

Reference:
http://libcloud.readthedocs.io/en/v0.14.0/compute/drivers/gce.html
ex_set_node_tags(node, tags)

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

amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests