Go with UTC by default
Go with the linux-server package by default
Introduce mirror_update configuration options

Initial support to configure the update repository for models/distribution.
This is initial implemented for Ubuntu preseeds.

Following options got introduced:

* mirror_update_host
* mirror_update_port
* mirror_update_path

By default all are unset and do not overwrite the default update repositories.
At least the mirror_update_host must be set to override the mirror configuration via preseed.
Introduce mirror_update option for Debian Wheezy preseed

Same as for the Ubuntu preseed ones.
Introduce mirror_update options for dtagcloud models