Split clustered tests to more detailed test cases

Split clustered tests to more detailed test cases

Benchmarking 3 node cluster was done by 4 test cases for
every combination of datastore (operational, config) and
shard leadership (leader, follower).
Due to bug 7390 more detailed test cases are prefered. Old test
cases were split to have more parameters configurable.

Also a simple leader stability check was added. Test simply compares
the leader ip at the end of the test with the one at the start.

Change-Id: Ie9fb6b5d8bf24aecdff034a106d358a1bcc0aefe
Signed-off-by: Peter Gubka <pgubka@cisco.com>