Add config option to override url for versions

The versions url returns the wrong data when glance api is behind
a proxy. This adds a new config option so it can be set properly.

DocImpact

Change-Id: I5ab53d608a6667435a4b03b0c832870716baaeb8
Closes-Bug: #1384379
Complete the change of adding public_endpoint option

Change-Id: I8190b102ab827a4cf4c23f0f350fae051c1d33f0
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>