Remove insecure default for signing_dir option.

The sample api-paste.ini file included an insecure value for the
signing_dir option for the keystone authtoken middleware.  Comment out
the option so that we just rely on the default behavior by default.

Fix bug 1174608.

Change-Id: I6189788953d789c34456bbe150b8ed6ce6f68403
(cherry picked from commit 58d6879b1caaa750c39c8e452a0634c24ffef2ce)