Implement periodic execution of benchmark scenarios

Being able to run benchmark scenarios not continuously but periodically,
setting the interval between two consecutive benchmark scenario launches,
is a crucial part of making Rally closer to real-world cloud usage scenarios.

Here we add such a capability by adding a new execution type, namely
"periodic". We also add new scenario config parameter "interval" which
enables the user to set the interval between two consecutive scenario
launches in minutes.

A sample benchmark config for periodical booting/deleting 5 servers, one
server every 2 minutes, now would look like this:

"benchmark": {
    "NovaServers.boot_and_delete_server": [
        {"args": {"flavor_id": 1,
                  "image_id": "73257560-c59b-4275-a1ec-ab140e5b9979"},
         "execution": "periodic",
         "config": {"times": 5, "period": 2,
                    "tenants": 3, "users_per_tenant": 2}}
    ]
}

Blueprint flexible-benchmark-config

Change-Id: I21a0a89f60bd5aaf9b3f419f72cdec7842bc7c38
Merge "Implement periodic execution of benchmark scenarios"