orchestra_proxy should be configurable

/var/lib/cobbler/snippets/orchestra_proxy
currently has:
# Point apt to orchestra-provisioning-server's squid
d-i mirror/country string manual
d-i mirror/http/hostname string archive.ubuntu.com
d-i mirror/http/directory string /ubuntu
d-i mirror/http/proxy string http://@@server@@:3128/

The values in there should be configurable.
It is quite likely that a different proxy would be desired, and/or a different mirror.

Thus, the user should be able to configure those without getting their changes overwritten on upgrade.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: ubuntu-orchestra-provisioning-server 2.27-0ubuntu1
ProcVersionSignature: User Name 3.2.0-8.14-virtual 3.2.0
Uname: Linux 3.2.0-8-virtual x86_64
ApportVersion: 1.90-0ubuntu1
Architecture: amd64
Date: Tue Jan 10 10:31:16 2012
PackageArchitecture: all
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: orchestra
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.orchestra.import.isos: 2012-01-09T23:35:37.653360