Add xvfbwrapper package for running Selenium tests headless

  - The requirement is related with an issue from Horizon project
  - The requirement needs the xvfb library installed

Change-Id: I93220d23a6838d0c29d264bfd683ee4693137c54
Remove amqplib from global-requirements

amqplib is not used directly by OpenStack, but only via
kombu. Newer kombu versions use amqp by default, so
any dependency on amqplib is obsolete and can be removed.

Change-Id: I58da17d05cb29e320d8956c9d03a66722beedea1
Merge "Add xvfbwrapper package for running Selenium tests headless"
Merge "Remove amqplib from global-requirements"