Fix duplicated keystone endpoints

Duplicated endpoints were created when updating 2 or more URLs.

Refactor keystone_endpoint to use prefetch and flush paradigms.

Closes-bug: #1234480
Change-Id: I91340995297c6afaf9c8841bc7ae71a8e25b1eb7
(cherry picked from commit ee7956e99a2979929fc09446929b60ca246821f6)