list_projects does forces admin interface

[Aleš Křivák]

Function list_project of authentication backend called during authenticate() always uses admin endpoint regardless of setting OPENSTACK_ENDPOINT_TYPE. Most of backends will inherit list_project from plugin.base.BasePlugin, where no interface (endpoint type) is given to created keystone client (and it has interface="admin" as default value).
Problem could be easily solved by passing interface as argument during authenticate(), but I am not sure wheter it would make sense for all possible authentication backends...