Solution 1:

Please note that cockpit exists in the official repository of Bionic so to install simply do:

sudo apt update && sudo apt install cockpit

Note: Remove those repos you added and the update via sudo apt update and proceed to install.

After due interaction with OP I discovered that the problem was he lacked the universe repos in sources.list so to overcome we followed these steps:

  1. Add repo:

     sudo add-apt-repository universe
    
  2. update accordingly:

     sudo apt update
    
  3. Now install:

     sudo apt install cockpit