Please create exitmap user/group on carinatum to run our exitmap tests there

We like to have a similar setup to our doctor running environment to finally get the exitmap scans running on TPO infra as well. We think carinatum would be a good host for that task and the setup should be similar to the doctor one. I am not sure about all of the things that need to happen for that on the sysadmin side but I guess we need at least:

1. Something like /srv/exitmap.torproject.org/ where our exitmap code and dependencies live
2. An exitmap user/group who was access to 1.
3. A way for @dgoulet and @gk to change into the exitmap user with something like sudo -u exitmap bash