snapshots: fix json type
snapshots: fix json type

We are generating a general object ({}), whereas Cassandra 2.1.x generates an
array ([]). Let's do that as well to avoid surprising parsers.

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@scylladb.com>

avikivity/scylla,raphaelsc/scylla,kjniemi/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,raphaelsc/scylla,duarten/scylla,duarten/scylla,kjniemi/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla