XenAPI: Always update apt sources

If an appliance is used as a base OS, the user might want to use that in
a different region. With this change we always update the used mirrors
in the template.

Change-Id: I7a119664efac1124e54064311c243c63c2a7944b
XenAPI: Fix vimrc

DevStack installs a default minimal .vimrc file for the stack user.
Unfortunately the `syntax on` config line is not recognised by the vim
installed in DomU. It results in an annoying message being displayed
whenever the user is using vi. To avoid this issue, removing the `syntax
on` line.

Change-Id: I224465cc3cdba3464ea0a9a751f250ecb6ddc9f3