policy.json is reloaded at each request

For every request, quantum v2 API invokes quantum's policy engine (quantum.api.v2.policy). Both policy checking routines, check and enforce, call init(), which reads the policy config file and resets the brain of the policy engine:

def init():
    global _POLICY_PATH
    if not _POLICY_PATH:
        _POLICY_PATH = find_config_file({}, 'policy.json')
        if not _POLICY_PATH:
            raise exceptions.PolicyNotFound(path=FLAGS.policy_file)
    with open(_POLICY_PATH) as f:
        _set_brain(f.read())

This is quite inefficient and might became problematic when the quantum server is under load.

The fix for this bug might consist in the same approach adopted for nova, where the file is reloaded only if its modification time differs from the cached version.