Use $() instead of ``

There are other instances of this in other files, just tidying
ceilometer at the moment.

Change-Id: I136a20d7ac50fdf02cbd1102613e324e313b7b0a
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
Merge "XenAPI: Always update apt sources"
Merge "Use $() instead of ``"