Change to use alembic to create db table schema

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
Merge "Change to use alembic to create db table schema"