Allows nova to read files as root
Allows nova to read files as root

 * Adds a rootwrap filter to limit readable files
 * Adds a utils method to read a file as root
 * adds tests to verify the additions work

Change-Id: Ic1ffb6f72f9b73795d5f39fac719842e6bdf16dd

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects