Add BSN plugin to agent migration script

Adds the Big Switch plugin to the migration script that creates
the agents table. A recent commit (d3be7b040eaa61a4d0ac617026cf5c9132d3831e)
added it to the agent tables but it missed this one so an error was thrown
during migration when it reached a table that depended on the agents table.

Closes-Bug: #1340405
Change-Id: I90a164a1b632c3794a7f7607f3f1da8f7d7b15db
(cherry picked from commit 2dd0a22fd6cc38cc27942572c8a19bf6a52fb32b)
Merge "Add BSN plugin to agent migration script" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack/quantum into stable/icehouse