Add CentOS background image to login screen.
    
    - Previously, the cockpit login screen was using its default background
      image, which is not the CentOS 8 artistic motif. As consequence, it
      affects the convention of one unique visual style in all visual
      manifestations. To make cockpit visual style consequent with CentOS
      visual style:
    
      - Add SOURCE/src/branding/centos/bg-plain.jpg holding CentOS 8
        artistic motif.
    
      - Update SOURCE/src/branding/centos/Makefile.am to consider bg-plain.jpg
    
      - Repack cockpit-185.1.tar.xz file with these changes.
    
      - Change cockpit.spec from 185.1-1 to 185.1-2.