users@openstack raises user unfriendly error message while trying to create tmp users without admin

[Walt Boring]

`users@openstack` context is designed to manage (create&delete) temporary users for workloads.
The attempt to create new users will be processed when the plugin configuration looks like:
   {"tenants": 1, "users_per_tenant": 1}

If there is no `admin` user configured for the environment the following error is raised that is not user-friendly at all:

2020-11-05 13:01:26.066 44142 ERROR rally.task.engine During handling of the above exception, another exception occurred:
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine Traceback (most recent call last):
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine File "/Users/i530566/devel/SAP/.rally-venv/lib/python3.8/site-packages/rally/task/engine.py", line 532, in _run_workload
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine with ctx_manager:
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine File "/Users/i530566/devel/SAP/.rally-venv/lib/python3.8/site-packages/rally/task/context.py", line 310, in __enter__
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine self.cleanup()
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine File "/Users/i530566/devel/SAP/.rally-venv/lib/python3.8/site-packages/rally/task/context.py", line 269, in cleanup
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine ctxlst = self._visited or self._get_sorted_context_lst()
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine File "/Users/i530566/devel/SAP/.rally-venv/lib/python3.8/site-packages/rally/task/context.py", line 219, in _get_sorted_context_lst
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine return [c(self.context_obj) for c in ctx_lst]
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine File "/Users/i530566/devel/SAP/.rally-venv/lib/python3.8/site-packages/rally/task/context.py", line 219, in <listcomp>
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine return [c(self.context_obj) for c in ctx_lst]
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine File "/Users/i530566/devel/SAP/.rally-venv/lib/python3.8/site-packages/rally_openstack/task/contexts/keystone/users.py", line 127, in __init__
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine self.credential = context["admin"]["credential"]
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine KeyError: 'admin'
2020-11-05 13:01:26.066 44142 ERROR rally.task.engine

[Andriy Kurilin (andreykurilin)]

The following part of Rally task triggers users context to try create temporary users. If you remove it, the existing users should be used:

```
"contexts": {
    "users": {
      "tenants": 1,
      "users_per_tenant": 1
    }
  },
```