Automatically lookup which server manager is used on each server
Automatically lookup which server manager is used on each server

This way we don't have to use different SRV records for ubuntu 14
and ubuntu 18 (idea from comments in #45).

We will always have a "failure" in the output on ubuntu 14 for the
command that checks if systemd is installed, but I found no way to
get around that. Part of the output displayed when deploying:

    00:03 deploy:lookup_server_service_manager
          01 command -v systemctl
          01 /bin/systemctl
        ✘ 01 deploy@localhost 0.003s
        ✔ 01 deploy@localhost 0.004s

twingly/capistrano-twingly