gce cluster: create nodes of same type in parallel
gce cluster: create nodes of same type in parallel

ex_create_multiple_nodes() supports to create nodes in parallel,
it directly saves about (db_nodes_number + loaders_number - 2) * 30s,
but the following setup will also be ahead of time, so it's
necessary.

Note: the suffix of instance names starts from `000`.

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

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