Remove install_data=/usr/share/cobbler/ from setup.cfg.

Having it makes it impossible to install into a different prefix (eg. /usr/local).
Make docpath relative.

Makes it possible to install into a custom prefix.
Make install path of tftpd.py relative

Makes it possible to install into a different prefix.
Merge pull request #667 from jansenm/mjansen/buildsytem-relative

Mjansen/buildsytem relative