Policy of manager api call

Methods in managers must be called via api.py.

- Rationality
  Isoration between managers
  Parameter checking
  More testable

Bad example
see https://bugs.launchpad.net/openstack-qa/+bug/872445

In this case, the compute manager directly calls volume manager without using it's API.
Because of this code the compute manager and the volume manager are not fully isolated.
This policy also affects input value validation policy (https://blueprints.launchpad.net/openstack-qa/+spec/nova-input-value-validation-policy).
Each of manager should validate input values. If the another manager call's internal method directly, we should validate all input values for all method in the manager because it's hard to know which api will be used.