Now that policy rules can be registered in code there is a desire to run
projects without a policy file. However oslo.policy assumed a policy
file would exist and would raise an error if it could not be found. This
changes that behavior to not error if a policy file is not found.
Because there are now tools which can generate policy files which list
the defaults, or list the effective policy there is no requirement that
a policy file be used in order to examine the policy that is in use. So
it should be possible to run without one.
Change-Id: Ia82df77f7a65aa1f3e3eaa7ed949103fa73fb603