Clustering: Add test to verify snapshot creation and read

Currently the Clustering Datastore tests create cars to verify datastore. However the size of data is not sufficient to create a snapshot. All cars data stays in journal logs. So any controller restart will read only from journal, and not from snapshots.

We need a new testcase to verify that creation and read of snapshots works correctly. The test should enable creation of snapshot when cars are added and then restart the controllers, which will test the read of snapshots. All data before the restart should be preserved via snapshots and journal logs.