Update login screen with CentOS background
    
    - Revert .cockpit.metadata to use c8 default .tar.xz file. The previous
      one was wrongly modified. We don't modify tar.xz, but patch them
      instead.
    
    - Add patches to change branding.css and Makefile.am.
    
    - Add SOURCES/centos-00-add-bg-plain-png.patch to consider bg-plain.png.
    
    - Add SOURCES/centos-01-update-login-screen-background-image.patch to
      change the background presentation in branding.css. It is now aligned
      to the top-right corner on the screen.
    
    - The semi-transparent remarking the login box was removed. To make the
      background image look better. This is also convenient because FreeIPA
      login screen is not using it either.
    
    - Update the SPEC/cockpit.spec file to add bg-plain.png source,
      patches and a change log entry.