MB-100 wait for bucket to warm before loading sample data

  - introduced new method ns_orchestrator:ensure_janitor_run
    that allows to wait for successful or unsuccsessful janitor
    run for particular bucket repolling if the bucket
    is warming up or janitor was killed to give way to
    another orchestrator command
  - I think the mystery of sleep(5000) line is solved now,
    so I removed it.

Change-Id: I59f9de05614b7aa720b3ebe16dd185c959470d4f
Reviewed-on: http://review.couchbase.org/28808
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>