Prevent possible server list damage in BigSwitch plugin

The old failover logic for the BigSwitch plugin modified a
list of controllers as it tested them. If the code for one
thread unexpectedly raised an exception or died, the global
server list could lose a server permanently.
This patch addresses that by flagging servers as failed instead
so the global server list is never modified.

Fixes: bug #1200022

(cherry picked from commit 3732f022778a7c4a47c58029ae77e32a0a4a6ee9)

Conflicts:

	quantum/plugins/bigswitch/plugin.py

Change-Id: Id2dcb820ef9f62fd03e3215bff3345e56c78afe2