remove linuxbridge plugin

remove linuxbridge plugin

This changeset removes the linuxbridge plugin, but retains the agent for ML2.
The database models were not removed since operators will need to migrate the
data.

Additionally, the ml2 migration script was altered to support Juno.  For
testing, a user must either run the migration against the icehouse
scheme or run the update, manually change alembic_version to juno and
then run the migration script.  Once the juno migration is added, this
manually step will not be required.

Change-Id: I70689b4247947e6dc08e80fd9b31da9dc691d259
Partial-Bug: 1323729