Split up repo setup and package installation after all to establish this
as a convention (its already done this way in the docker role and was
done this way in the nginx role before) to highlight that an external
repo is used.
We always just want the latest anyway and therefore don't use it, so no
need to keep the complexity introduced by that setting.
Also merge repo_setup and nginx_install task lists into one
nginx_install task list as keeping two files isn't necessary.
Finally improving naming a bit.