Change Jepsen tasks to run on the ubuntu1604-build distro in Evergreen

After the changes from SERVER-33805, we no longer use the LXC containers in order to do network partitioning. We can remove the special casing for the ubuntu1404-jepsen distro from the etc/evergreen.yml project configuration by moving it another build variant that has lein, gnuplot, and JDK 8 installed.