XenAPI: Always update proxy settings

If a user wants to re-use an xva just with different proxy settings, he
can now do it, as this change will always update /etc/apt/apt.conf to
reflect the settings in the user's localrc.

Change-Id: I8a6e9c88304bc887ea8269d946e89a5ba258b126
Merge "XenAPI: Always update proxy settings"