Add support of stress testing in current "continious" type of execution

Currently we have type of execution "continuous" where we are specifying "times" or "duration" + amount of "active_users".

It is great in case if you are interested how OS will work on some load.
But in case when you would like to get maximal load that handle cloud it is a bit painful.

So at this moment you should write something like:

   "benchmark": {
        "NovaServers.boot_and_delete_server": [
            {"args": {"flavor_id": 1, "image_id": "73257560-c59b-4275-a1ec-ab140e5b9979"},
             "execution": "coninious",
             "config": { "active_users": 10, "times": 100, "tenants": 3, "users_per_tenant": 2}},

            {"args": {"flavor_id": 1, "image_id": "73257560-c59b-4275-a1ec-ab140e5b9979"},
             "execution": "coninious",
             "config": { "active_users": 15, "times": 100, "tenants": 3, "users_per_tenant": 2}}

           ......

            {"args": {"flavor_id": 1, "image_id": "73257560-c59b-4275-a1ec-ab140e5b9979"},
             "execution": "coninious",
             "config": { "active_users": 100, "times": 100, "tenants": 3, "users_per_tenant": 2}}

        ]
    }

And there is no way to stop on some amount of active_users if failure rate is X.

So this blueprint is attempt to simplify and automate this case.

New config should look like:

I would like to have a "stress" type of execution. E.g.:

    "benchmark": {
        "NovaServers.boot_and_delete_server": [
            {"args": {"flavor_id": 1,
                      "image_id": "73257560-c59b-4275-a1ec-ab140e5b9979"},
             "execution": "stress",
             "config": {
                     "max_failure_rate": 0.1, "active_users": {"start": 10, "end": 100, step: 5},
                     "times": 200, "tenants": 3, "users_per_tenant": 2}}
        ]
    }

And it will make the same as previous. So run N benchmarks raising amount of active users with specified step. And if failure_rate is bigger then max_failure_rate it stops.