Add DB support for resource usage tracking

Add DB support for resource usage tracking

This patch introduces database support for tracking Neutron
resource usage data. A single DB model class tracks usage
info for all neutron resources.

The patch also provides a simple API for managing resource
usage info, as well as unit tests providing coverage for
this API.

This patch also makes a slight change to the ContextBase
class, adding the ability to explicitly set is_advsvc at
initialization time. While this probably makes no difference
for practical use of the context class, it simplifies
development of DB-only unit tests.

Related-Blueprint: better-quotas

Change-Id: I62100551b89103a21555dcc45e84195c05e89800