Bug 10944: fix mixed content warnings for covers on staff client

Bug 10944: fix mixed content warnings for covers on staff client

This switches the URLs used to access Amazon's cover images to the
SSL image server, preventing warnings when using this on a secure
 site.

To test:
* Enable Amazon images in the sysprefs, all other cover images off.
* Do a search in the staff client. Using firebug or similar, inspect the
  images that indicate both "no image found" and the covers.
* Are they all https, and showing correctly?

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>