Removes outdated targets from tox.ini

Cleaning up some leftovers tox targers after the recent
refactoring. This patch also removes the `[testenv:scenario]` target
since there won't be any case where we require running all the tests
at once. Each test needs a vastly different environment to run, so
there won't be, I think, any situation where running all of them is
relevant.

Change-Id: Ida8858c506e6f6678a147e1753c49a94f170ddd4

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.3.1
-envlist = flake8,py27,py35
+envlist = linters,py27,py35
 skipsdist = True
 
 [testenv]
@@ -28,9 +28,3 @@ exclude = .git,.venv,.tox,dist,doc,*egg
 
 [hacking]
 local-check-factory = tempest.hacking.checks.factory
-
-[testenv:scenario]
-sitepackages = True
-commands =
-  find . -type f -name "*.pyc" -delete
-  tempest run --serial --regex '(^whitebox_tempest_plugin\.tests\.scenario)' {posargs}