distrib: Add script to install gitblit on fedora

The script will create a environment file keeping all the major settings and
installs a systemd unit file.
The script (mainly the unit file part) should also work for other systemd
based distributions like ArchLinux. But /etc/sysconfig may not exist there.

Signed-off-by: Soeren Grunewald <soeren.grunewald@desy.de>