Refactor BigSwitch error handling to use db rollbacks

This patch adjusts most of the logic handling the
calls to the controller in the BigSwitch/floodlight
plugin to make use of the db rollbacks from
sqlalchemy for free on exceptions. This eliminates
several complex try-except blocks and makes
maintaining db<->controller consistency easier.

Fixes: bug #1215823
Change-Id: Ia636c40e744b3b1c543e891791bf492df4f675d2