[Sahara] Copy policy.json if it exists

To enable policy check in Sahara we need gate tests passed.
Tests will not pass until Sahara has default policy.json (
e.g. http://logs.openstack.org/09/131609/1/check/check-tempest-dsvm-full/536d5e7/console.html). 
New code will copy of policy.json if it exists.

So, for now this code will do nothing.
Once policy.json appears in etc/sahara, the code will start coping
it to sahara conf dir. This will allow to merge auth support with
gate tests passed.

Related blueprint: auth-policy

Change-Id: I98e108ff02aacb91570f97e457d67dd02779ae3d
Merge "[Sahara] Copy policy.json if it exists"