Set BigSwitch plugin to use existing host database model

The BigSwitch plugin had a data model duplicating one
that was recently added into the base portbindings db
file. This patch removes the BigSwitch model and
updates the BigSwitch plugin to reference the plugin-
agnostic model.

Fixes: bug #1211641
Change-Id: I1fdbf8690e71fb2d6e9d6a6dbd0571fa2c32a7e0
Merge "Set BigSwitch plugin to use existing host database model"