The latest selenium webdriver fails with old geckodriver.

This patch also fixes horizon-selenium-headless job.

Change-Id: I8dfdc7e80dfe680d6699fc7f1f8325b85a9f7c8f

This patch leave selenuim-headless non-voting since it was broken for a
while so we should be sure that it's stable enough.

Related blueprint: improve-horizon-testing
Change-Id: Ic2a877a4eefc2f10fb25e64c387b81fc18302a2a