Enforce usage of specific endpoint in providers

This change ensures that all requests to Keystone API use the specific
endpoint, instead of the public endpoint which the openstack client
uses by default.

Change-Id: I33520272b04c908f25eb145ae5db21e6bc165c99

lib/puppet/provider/keystone.rb

@@ -48,6 +48,10 @@ class Puppet::Provider::Keystone < Puppet::Provider::Openstack
         creds['region_name'] = conf['region_name']
       end
 
+      if conf['interface']
+        creds['interface'] = conf['interface']
+      end
+
       return creds
     else
       raise(Puppet::Error, "File: #{conf_filename} does not contain all " +