Improve Proxy Signing Directory Support

1. The signing_dir was being created on every swift node
when it's only needed on proxies:

  http://docs.openstack.org/grizzly/openstack-compute/install/apt/content/installing-and-configuring-the-proxy-node.html

2. The signing_dir was not configurable, even though
the OpenStack doc's show multiple references
(/var/cache/swift and /home/swift/keystone-signing).

Based off the following patch from master:
  https://review.openstack.org/#/c/40356/

Change-Id: I3eb00046fc535a32d7a1800d15489e9fe071373b