Registers in-code the last remaining policy rules.
Adds missing 'discoverable' rules. Without them,
the extension_info API can fail, as it tries to check the
os_compute_api:os_server_tags:discoverable rule. As it wasn't
previously registered, when listing the available extensions,
an exception of type PolicyNotRegistered is encountered.
In order to validate this, functional/api_sample_tests/test_extension_info.py
now runs without mocking policy.authorize.
Switches extension_info to context.can.
Switches nova.cells.filters to context.can.
Switches network.neutronv2.api to context.can.
Removes the rest of the entries in etc/policy.json.
Removes DefaultPolicyTestCase, as it tests the default
policy rule, which is not registered.
Removes rules from fake_policy.py that brings no value,
that are the same as the default values.
Removes extensions authorizer factories.
Removes nova.policy.enforce.
Change-Id: Ie7771768f4f3efe0edc787c12f297aa93d533d7e
Partially-Implements: bp policy-in-code
7d01bceaa0