Corrects proxy server conf in swift install guide

The install and configuration guide for object storage has
proxy-server.conf contents with /home/swift/keystone-signing as
signing directory. Without the directory being available swift proxy
service is unable to start. Signing directory is a temporary cache
directory which defaults to /tmp. Hence removing this entry makes
/tmp as default signing directory. This patch removes signing_dir
option to make swift use /tmp as signing directory implicitly.

Change-Id: I8b9b70b30af2767f48705acb14329833179b96fc
Closes-Bug: #1329798
(cherry picked from commit abed3e59e159136b39a498a013983d0cf9efded9)