Adds Database Configuration Support for Havana

Previously, database configuration information was stored in the
plugin .ini file.  As of Havana, a [database] section has been
created in neutron.conf to manage the database configuation for
all Neutron plugins:

https://github.com/openstack/neutron/blob/stable/havana/etc/neutron.conf#L315

Additionaly, several database flags have changed from Grizzly to
Havana.  For backwards compatibility, the previously supported
database flags are contained in neutron::server with a default
of false.

Change-Id: I300fc12092bca1abdd105e0c0c272f7fd49258ac