BigSwitch ML2: Include bound_segment in port

Includes bound segment in port create/update
requests to disambiguate when the parent network
has multiple segments.

Closes-Bug: #1294355
Change-Id: I59ef38aa5de46c1229384e0c4a9f0ee01c1bc678
add HEAD sentinel file that contains migration revision

This change wraps the command to generate a migration with code to
update a file called HEAD. The HEAD file will contain the revision ID
of the head of the migraton timeline. Additionally, check migrations has
been altered to verify the contents of this file against the timeline
head.

This file helps the OpenStack gate detect potential migration branches
without running alembic via git merge conflicts.

Closes-Bug:1288427
Change-Id: If382c57baea061753d3f4fcd6faec1a31fbfb7ed
Merge "add HEAD sentinel file that contains migration revision"
Merge "BigSwitch ML2: Include bound_segment in port"