Use cls instances instead of object in benchmarks

Using @classmethod as a base for scenarios was actually caused
by historical reasons (of using pytest as a base for benchmarks)
As we don't use it anymore (for a long time) we are able to simplify
benchmark framework and use cls instances and plain methods.

This is only the first step. That just make required changes and fix
unit tests. In next patchset I will refactor & clean exisitng unit tests.

blueprint refactor-scenarios-to-use-class-instances

Change-Id: Idd270635f70731ccf575174f8f96dc1e95cc0f8a
Add rally use command

This adds a new group of commands : 'rally use'
the first implemented is 'rally use deployment <deploy_id>'

As it is not possible in python to export globaly an env var,it saves the
variable in a file at ~/.rally/deployment like :
RALLY_DEPLOYMENT=1234abcd

For each command that takes a deploy_id as parameter, this
parameter is now optional.

The algorithm to find the deploy_id is as follows :
- If the --deploy-id param is set, use it
- If the environ contains the key 'RALLY_DEPLOYMENT', use it
- load the content of ~/.rally/deployment and if it contains the key
RALLY_DEPLOYMENT, use it
- Else, raise an InvalidArgumentException

Note that I have not been able to use the default arg in @cliutils.args
because it is called before the effective method call. Setting
deploy_id=envutils.default_deployment_id() has the same problem

Partially implements blueprint: rally-use-command

Change-Id: I1e62e26d259bd9e81f1c008058ee9ce91cc3290c
Create rally deployment from OS_* environment variables

This saves users from creating dummy config json files when they already
have an openrc for use with the other openstack clients.

Change-Id: I487af60e0c6a5eadfc12f04fbba9c4a4186740e0
Trello: https://trello.com/c/5ZkMb9zL