neutron removed the capability to auto generating the table schema and
use the alembic as the only way to create the db tables. powervc neutron
agent needs to follow the same way to create the tables schema and it
also added the db schema migration capability for powervc neutron agent.
As db2 alembic has problem at this time, I still retain the auto create
the table schema during startup, once db2 alembic is enabled, then we
can totally switch to use alembic to create the db schema
Change-Id: I4b5982b2f67b33aae3b8acc65b91fc6247b9b8a4