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
(cherry picked from commit 9b2b5e1482ebec818556946230066984bf647186)
Pass object to policy when finding fields to strip

During the evaluation of fields to strip in responses to list
operations, pass also the first object in the list to the
policy engine.
This will avoid errors in policy evaluation if during
an upgrade from icehouse policy.json was not updated to remove
attribute-level policies dependent on resource values.

Closes-Bug: #1323715

Change-Id: Iaa6ed3bbf4a07ce0b614a4197cabdfa1cb36d427
(cherry picked from commit 2bd97d205b98292be709b0257bda5fc489eb643a)
Merge "Perform policy checks only once on list responses" into stable/icehouse
Merge "Pass object to policy when finding fields to strip" into stable/icehouse