Improve User context

*) Default value for concurrency should be at least 30.
*) Change templates for names of users and context
*) Store tenant_id in user (it's very usefull in many cases):
role context:
https://review.openstack.org/#/c/87984/4/rally/benchmark/context/role.py
quotas benchmarks:
https://review.openstack.org/#/c/87283/7/rally/benchmark/scenarios/quotas/quotas.py
(where we should setup quotas for passed user)

Change-Id: I00a97cb6ba20f6ee1636e56dc9ad35216ed30144
Merge "Improve User context"