Thi patch skip integration tests which fails on gates until specified
bug will be resolved.

@decorators.skip_because doesn't work somewhy so I used unittest.skip
instead of it. I'll fix skip_because decorator in a follow up patch.

Related-Bug: #1792028
Change-Id: Ida79b5064c2457afc15db6e63a45478d711e02b5

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