Delete expired instance console auth tokens

Instance console auth tokens never get deleted from the cache
before the instnace is deleted, this is a waste of memory. now
we check if tokens are expired then remove them from the cache
before store new tokens.

Fix bug #1209134

Co-authored-by: Takashi Natsume <natsume.takashi@lab.ntt.co.jp>
(cherry picked from commit e3bbf2e81137bbcdc39bdfb4c4ff62f30c37afda)
Conflicts:
	nova/tests/consoleauth/test_consoleauth.py

Change-Id: I8dd5089ebaed3b3d91932f1f1558bbe302cd5675
Merge "Delete expired instance console auth tokens" into stable/grizzly