Add policy sample generation

This adds the entry point and config file necessary for using the
oslo.policy sample generation script. It also adds a tox target to
simplify the usage of it.

The command is also run in the docs tox target since the sample file
can be considered documentation.

Change-Id: I18a1fa567fc04b0e0a37c02f20024b2ce483f060
Partially-Implements: bp policy-in-code

setup.cfg

@@ -36,6 +36,13 @@ oslo.config.opts =
 oslo.config.opts.defaults =
     nova.api = nova.common.config:set_middleware_defaults
 
+oslo.policy.policies =
+    # The sample policies will be ordered by entry point and then by list
+    # returned from that entry point. If more control is desired split out each
+    # list_rules method into a separate entry point rather than using the
+    # aggregate method.
+    nova = nova.policies:list_rules
+
 nova.compute.monitors.cpu =
     virt_driver = nova.compute.monitors.cpu.virt_driver:Monitor
 nova.compute.resources =