ML2: database needs to be initalized after drivers loaded

Previously, if you started neutron-server with an empty database some
of the tables that drivers use are not automatically created. That said,
one should probably run neutron-db-manage manually to create the tables
and not rely on neutron to do this. This regression was cause in 326b85.

Change-Id: I2c578733de0213945b31fba86a3b0ea45c02295a
Closes-bug: #1285993
Co-Authored-By: Itsuro Oda <oda@valinux.co.jp>
Merge "ML2: database needs to be initalized after drivers loaded"