Fix Migration 50e86cb2637a and 38335592a0dc

When the rename of quantum->neutron occurred here ee3fe4e8 it also renamed
the the table creation from quantum_nvp_port_mapping to
neutron_nvp_port_mapping. This went undetected for a long time because
when neutron-server starts up it pushes down the scheme for tables that
are not there so the table would be created.

Because of this the following migration 50e86cb2637a called
op.rename_table('neutron_nvp_port_mapping', 'neutron_nsx_port_mappings')
though the table name being used was quantum_nvp_port_mapping. Because of this
the quantum_id->nvp_id mapping was never migrated over to the new table and
you would be left with a quantum_nvp_port_mapping table hanging around.

In addition, the downgrade would rename the table to neutron_nvp_port_mapping
instead of quantum_nvp_port_mapping. This patch addresses this issues.

Change-Id: I4f80b7b9dc56996ecd83826ee65918f5311c7c4f
Closes-bug: #1267619
Merge "Fix Migration 50e86cb2637a and 38335592a0dc"