Add quota per-tenant.

Add quota per-tenant.

blueprint quantum-api-quotas

We implement it as an extension for linux bridge and ovs plugins.
We also expose the /quotas/Xx url to client to operate the quota.
We need admin role to show other tenant's quota, and to update quota data.
Any user can show its own tenant's quota. An DB table is used to save the
quota for each tenant.

To use it, we have in quantum.conf:
quota_driver = quantum.extensions._quotav2_driver.DbQuotaDriver

The default quotas for each tenant are defined in quantum.conf too.

In addition, modify extension framework to allow exposing a new resource and
its controler. The extension can check the environment, such as configuration
in global cfg.CONF to decide if it can be enabled.

Also, we can define enabled extensions for each plugin in extensions.py
New resources can be put into quota framework via quota_items in nova.conf

Change-Id: I54d6107fdb2808cdae1a40b501ed8c7f379dedee