Add looped mode to bulk operation

[gbanasiak]

Adds looped mode to bulk operation. That is useful in benchmarks where we want Rally to ingest for a specific amount of time even if corpus is not big enough to sustain indexing at a specific rate for this time.

This mode is meant to be used together with time-period or iterations at the task level. Otherwise Rally will never complete the task.

Example use:

{
  "name": "some-bulk-task",
  "operation": {
    "operation-type": "bulk",
    "bulk-size": 5000,
    "looped": true
  },
  "time-period": 300,
  "target-throughput": 2
}

[gbanasiak]

Re-running IT test failure of which seems unrelated.