Added schema interface to datasource drivers

Previously, the Nova and Neutron datasource drivers had
different ways of exporting their schemas.

This change introduces an interface for exposing a schema.
The interface consists of one method, 'get_schema', which
returns a dictionary that maps each tablename to a tuple
of column names.

Change-Id: I21d6f5926560a2e720a4e8240381237bdacdfe70
Merge "Added schema interface to datasource drivers"