Provide 'setup' command for refstack-client

'setup' command provides setup Tempest test runner from tagged
release. User is able to specify Tempest git repo, tag in it and a
target directory for Tempest. Virtual environment for Tempest  is installed
into Tempest target directory. So now, refstack-client is able to have a
few installed different versions of Tempest.

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

Change-Id: I182c3870a7ce4bbcb188836d189538dfd54b1c26