use graduated oslo.policy

- Add oslo.policy to requirements.txt
- Remove in-tree copy of oslo.policy

Change-Id: Ia6fef939cfe0df33476ccee365934a5d2878f90d
Partial-Bug: #1458945

setup.cfg

@@ -36,7 +36,6 @@ oslo.config.opts =
     nova.scheduler = nova.scheduler.opts:list_opts
     nova.virt = nova.virt.opts:list_opts
     nova.openstack.common.memorycache = nova.openstack.common.memorycache:list_opts
-    nova.openstack.common.policy = nova.openstack.common.policy:list_opts
 
 nova.compute.monitors.cpu =
     virt_driver = nova.compute.monitors.cpu.virt_driver:Monitor