Add scenario tests to Tempest

This blueprint proposes to add a new category "scenario tests" in Tempest.
Scenario tests are for testing across the components such as Nova, Keystone, Glance and so on.
By scenario tests, developers will be able to check whether a new code (bug-fixes or features) will cause a side effect to the other components.
We will be able to check OpenStack basic functions by common use case scenarios.
I'd like to propose a minimum basic scenario test that must work at any OpenStack environments.

Implementation
===============
* add tempest/tests/scenario directory
* add scenario tests in the directory like following

"""
Scenario-1: very small cloud, basic life-cycle scenario, single-flavor, single-image, single-tenant...
- for admin user
1. Create a flavor
2. Create a image
3. Configure network settings
4. Create & configure a member role, a project, a quota, a user
- for regular user
5. Create a key-pair
6. Boot a instance. List & show the instance
7. Create a volume. List & show the volume
8. Attach the volume
9. Detach the volume
10. Delete the instance
- for admin user
11. Delete the user, the role, the network, the image, the tenant, and the flavor
"""