Perform policy checks only once on list responses

Perform policy checks only once on list responses

The policy engine is currently being called for every attribute
of every resource to be returned by a list response. This is
harming the API performance; moreover such a high number of checks
is also unnecessary.

This patch therefore slightly changes the API logic so that list
response first determine the list of attributes which should be
returned querying the policy engine and then use this list for
all resource items to be returned.

To this aim a few methods in base.py needed to be refactored.
This patch also removes the routine check_if_exists from policy.py
and the related PolicyNotFound exception.

Finally, this patch also removes unnecessary admin_or_owner rules
when applied to attributes. This kind of rule indeed has no effect
anyway because of Neutron's ownership checks. The rules were removed
because this change won't allow anymore for having attribute-level
policies whose evaluation result depends on the resource value.

Implements blueprint faster-list-responses

Change-Id: I21b8273add5d5984f512ad94af5a99cf0b0a5d93