Comment 15 for bug 1378732

In my comprehension, this script has two mainly use, one is to migrate the db from openvswitch plugin to ml2 plugin on icehouse release, and another is to migrate the db from openvswitch plugin on ichouse release to ml2 plugin on juno release.

1.I use devstack to intall an all-in-one openstack(icehouse) as my test environment, and the default core_plugin is openvswitch. I follow the comment in the migrate_to_ml2.py to move it to ml2, and it works for me, cmd is:
python -m neutron.db.migration.migrate_to_ml2 openvswitch mysql://root:lee0612@127.0.0.1/ovs_neutron?charset=utf8 --release icehouse --save-tables True

2.To uppgrade db from openvswitch on icehouse release to ml2 on juno release, we hava two ways:
    1)separate it in two steps, first migrate openvswitch plugin to ml2 plugin on icehouse, and then use neutron-db-manage to upgrade it to the latest head juno, this works, cmd is:

    python -m neutron.db.migration.migrate_to_ml2 openvswitch mysql://root:lee0612@127.0.0.1/ovs_neutron?charset=utf8 --release icehouse --save-tables True
    neutron-db-manage --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugins/ml2/ml2_conf.ini upgrade head
    2) upgrade the db from icehouse to juno first, and then migrate openvswitch to ml2, this is how we get this bug. and I have a little confusion on this solution, as we know, juno do not hava openvswith plugin and linuxbridge plugin, they hava been replaced by ml2, so migrating from openvswitch, which is not exist, to ml2 on juno seems odd. Cmd is:

    neutron-db-manage --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugins/ml2/ml2_conf.ini upgrade head
    python -m neutron.db.migration.migrate_to_ml2 openvswitch mysql://root:lee0612@127.0.0.1/ovs_neutron?charset=utf8 --release juno --save-tables True(this failed)

    I also push a patch to stable/juno to solve this bug.
    https://review.openstack.org/#/c/166725/