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
Change-Id: Id2dcb820ef9f62fd03e3215bff3345e56c78afe2
Merge "Prevent possible server list damage in BigSwitch plugin"