Use osapi_glance_link_prefix for image location header

When an image is created the location header returned is generated based
on the request url that nova-api receives.  If SSL termination happens
upstream the request url is an http request, and the location header is
wrongly generated with that.  The osapi_glance_link_prefix config option
is available for almost precisely this purpose, so let's use it.

Change-Id: I24e46ed8fc0763f9ddeec65e0d79590dd7b86bef
Closes-bug: 1298005
Merge "Use osapi_glance_link_prefix for image location header"