Create/delete sandboxes from the context

[jsitnicki]

@jtaleric points out that Rally scenarios should be autonomous, that is they should not depend of the system state created by any scenarios that have run before them.

This is currently not true for scenarios that Rally ovs plugin provides because they depend on having a running controller sandbox (one with ovn-northd and NBDB/SBDB databases). Also, scenarios that involve port binding assume that there are fake chassis sandboxes running.

Instead of having scenarios for creating/destroying sandboxes that we need to run before/after the actual workload scenario, it would make sense to extend existing context(s) or create a new one that will create sandboxes before the scenario runs and clean them up afterwards.

An example of a such context can be found in Rally openstack plugin:
https://github.com/openstack/rally/blob/master/rally/plugins/openstack/context/nova/servers.py

[l8huang]

Make sense. Could you please create a new context for setup/teardown sandbox before/after run scenario.

For scalability test, we need to control how many sandbox should be setup before run scenario and keep them after done.

[jsitnicki]

@l8huang we'll make sure that current behaviour is unaffected by any changes to contexts.