Nexenta support

I see there have been made a few changes, compared to branch 2.4.

Can you tell me, what is the preferred way of putting extra info to the /var/lib/tftpboot files? I have install_profiles/ copied over from the kickstarts/ directory; this is likely to be a required step for booting Nexenta. In the file cobbler/pxegen.py, lines 196-199. Those paths are currently hardcoded which is probably not the best practice.

Also, my tests on branch `master` are actually failing, so help on QA would be much appreciated.