Setup_env script should install Tempest from tag

setup_env script should be able to install Tempest from specific tag
release. Key -t added to setup_env script. It specifies Tempest tag
release which will be installed as a test runner. By default last tag
from Tempest github repository will be used. Installed by setup script,
Tempest test runner is only single available option for now. Because of
it, option --tempest-dir option is removed from refstack-client

https://storyboard.openstack.org/#!/story/309
https://storyboard.openstack.org/#!/story/324

Change-Id: I182c3870a7ce4bbcb188836d189538dfd54b1c26