Network RBAC DB setup and legacy migration

Network RBAC DB setup and legacy migration

This patch implements the database model required for the network
RBAC work. In addition it migrates the current network and subnet
'shared' attributes to leverage the new table.

'shared' is no longer a property of the DB model because its status
is based on the tenant ID of the API caller. From an API perspective
this is the same (tenants will see networks as 'shared=True' if the
network is shared with them). However, internal callers (e.g. plugins,
drivers, services) will not be able to check for the 'shared' attribute
on network and subnet db objects any more.

This patch just achieves parity with the current shared behavior so it
doesn't add the ability to manipulate the RBAC entries directly. The
RBAC API is in the following patch.

Partially-Implements: blueprint rbac-networks
Change-Id: I3426b13eede8bfa29729cf3efea3419fb91175c4