Enable different tenants in the scenario runner

The current implementation of the scenario runner runs all the benchmark
scenarios from the admin account returned in the endpoints config from the
deploy engine. It uses thus only one tenant and one user to emulate all the
load on the cloud which is a bit unrealistic.

This patch allows the user to add "tenants" and "users_per_tenant" parameters
to the test config, like:

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

The scenario runner then creates the necessary amount of temporary tenants and
users and uses them randomly while executing benchmark scenarios.

Blueprint test-engine

Change-Id: I5a85a344ed73242d5b33f7af88e222c049f95dd4
Merge "Enable different tenants in the scenario runner"