This patch allows administrators to set disk_formats only for glance,
while horizon will retrieve list of supported formats from glance API.
IMAGE_BACKEND_SETTINGS still may be used to redefine display name
of the format or additionally limit list of availble ones.
Change-Id: Ia4ea513023895f4ad2a87f91e3d2837c7668d9ae
Closes-Bug: 1853822