Logout after each Behat scenario

In newly written tests we were already doin this manually in each
scenario. Between scenario's the user manager in the DrupalExtension was
not correctly keeping track of the logged in user status which could
prevent a new scenario from logging a user in.

With the removal of clean-up in scenario's we now need that logout step
literally everywhere, so it's easier to add it as an `@afterScenario`
and run it every time than to require developers to decide about 100s of
places whether it's needed.

See jhedstrom/drupalextension#641

tests/behat/features/bootstrap/FeatureContext.php

@@ -698,6 +698,9 @@ public function openFileAndExpectAccess($fid, $expected_access) {
     /**
      * Log out.
      *
+     * Until https://github.com/jhedstrom/drupalextension/issues/641
+     * @afterScenario
+     *
      * @Given /^(?:|I )logout$/
      */
     public function iLogOut()