Adds nova-policy-check cmd

This patch adds new command line 'nova-policy'. This is administrator
command line, it is tool for experience the policy discovery which
is still under-development.

Co-Authered-by: Alex Xu <hejie.xu@intel.com>

Change-Id: If4abf0fca043de2cb5c719a53505526f3c0b871b
Implements: blueprint discoverable-policy-cli

setup.cfg

@@ -64,6 +64,7 @@ console_scripts =
     nova-manage = nova.cmd.manage:main
     nova-network = nova.cmd.network:main
     nova-novncproxy = nova.cmd.novncproxy:main
+    nova-policy = nova.cmd.policy_check:main
     nova-rootwrap = oslo_rootwrap.cmd:main
     nova-rootwrap-daemon = oslo_rootwrap.cmd:daemon
     nova-scheduler = nova.cmd.scheduler:main