Horizon login page contains DOS attack mechanism

the horizon login page (really the middleware) accesses the session
too early in the login process, which will create session records
in the session backend.  This is especially problematic when non-cookie
backends are used.

Change-Id: I9d2c40403fb9b0cfb512f2ff45397cbe0b050c71
Closes-Bug: 1394370
(cherry picked from commit 61d09f6f96a22cd6c0ade58f6486cdbd118c5e2a)

Conflicts:
	horizon/middleware.py