system_keyspace::get_topology_request_entries returns entries for
requests which are running or have finished after specified time.
In task manager node ops task set the time so that they are shown
for task_ttl seconds after they have finished.
Add topology_tasks test suite for testing task manager's node ops
tasks. Add TaskManagerClient to topology_tasks for an easy usage
of task manager rest api.
Write a test for bootstrap, replace, rebuild, decommission and remove
top level tasks using the above.
