Update migration scripts to support DB2

Three of the migration scripts are causing failures with DB2.

- DB2 doesn't support nullable column in primary key

- Hard coded SQL statements which use False/True as Boolean arguments
  are not compatible with DB2. In DB2, Boolean columns are created as
  small integer with a constraint to allow only 0 & 1.

- Hardcoded update rows from other table sql is not compatible with DB2

- Foreign key constraints require additional handling

Co-authored-by: Rahul Priyadarshi <rahul.priyadarshi@in.ibm.com>
Change-Id: I82e2d1c522b81fed90a1e5cc6f2321f80797cf7b
Closes-Bug: #1328019
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
Fix pid file location to avoid I->J changes that break metadata

Changes in commit 7f8ae630b87392193974dd9cb198c1165cdec93b moved
pid files handled by agent/linux/external_process.py from
$state_path/external/<uuid>.pid  to $state_path/external/<uuid>/pid
that breaks the neutron-ns-metadata-proxy respawn after upgrades
becase the l3 or dhcp agent can't find the old pid file so
they try to start a new neutron-ns-metadata-proxy which won't
succeed, because the old one is holding the port already.

Closes-Bug: #1376128
Change-Id: Id166ec8e508aaab8eea35d89d010a5a0b7fdba1f
Merge "remove linuxbridge plugin"
Merge "Update migration scripts to support DB2"
Merge "Fix pid file location to avoid I->J changes that break metadata"