Horizon integration tests require Xvfb

Xvfb isn't installed by default on devstack nodes. Since it's a
requirement for the Horizon integration tests, create a new macro to
install it only for the jobs where it is needed.

Change-Id: I168fe5f1e6253ded89728ee2a69f8cf3197d2000
Merge "Horizon integration tests require Xvfb"