Ensure a fully functional cobbler in 16.04 and 16.10

[Impact]

Cobbler (2.4.1-0ubuntu2) does not work as installed.

https://bugs.launchpad.net/ubuntu/+source/cobbler/+bug/1570891

... more to be listed.

[Test Case]

# apt-get install cobbler
# cobbler check
- returns an error in failing versions, runs successfully (with appropriate messages) in working version
# cobbler get-loaders
- returns an error in failing version, runs successfully (with appropriate messages) in working version
# cobbler signature update
- returns an error in failing version, runs successfully (with appropriate messages) in working version

[Regression Potential]

The version of cobbler shipped in 16.04 (2.4.1-0ubuntu2) currently does not work OOB. And the codebase it is based on is so old as to be unmaintainable. When Ubuntu packaged this version of Cobbler originally (14.04), MAAS actively used Cobbler. However, it no longer does and cobbler is now in universe. I believe we are best-served by providing an update to the release notes (which I will provide upon acceptance of this SRU) for 16.04 and shipping an updated and working version (which would also be in-sync with Debian). Finally, there is probably not valid upgrade path for 2.4 Cobbler to 2.6 Cobbler (again, will be documented in the release notes), due to a number of settings and configuration options having changed.

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Note testing is needed to ensure the 2.6.6+dfsg1-12 version is actually fully functional (I've already found and submitted one bug to Debian).

Ideally, Debian will resolve https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=813549 and we can sync that version instead (which doesn't have the aforementioned bug).

Also, the upgrade path for users who already installed 2.4.1-0ubuntu2 needs to be well-documented, as this might break those users (on 16.04).