Change benchmark config format

The patch improves the benchmark config format in that it moves some technical
parameters for each scenario run (like "times", "tenants" etc.) in a special
"config" section and also renames the "concurrent" parameter to "active_users",
which is way more clear for the end-user.

A typical benchmark config now would look like this:

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

The new mandatory "execution" parameter indicates the means of scenario
execution and can now be set to only one value, namely "continuous". Another
execution type, "periodic", will be implemented in another patch.

The patch also improves unittest coverage for possible benchmark config issues
and replaces some nested mock calls with a better wrapper syntax.

Blueprint flexible-benchmark-config
Change-Id: Iabc90838aa8ad9cb44d73892852ed21c652ad274