Fix race when running initialize_gateway_device()

If multiple calls that result in initialize_gateway_device() being
run occur at roughly the same time then there is a race between the
ip route commands being run at the same time. This will cause
instances to go into an error state. This commit adds a global lock
to the initialize_gateway_device() method to prevent it from being
run at the same time to avoid this issue.

The race condition is not directly testable in unit tests because it
requires a multithreaded environment to run
initialize_gateway_device() at the same time. It was uncovered
with tempest in parallel.

Fixes bug 1217163

Change-Id: Ib750381636d1341062928d0abc8d3518e327935e

nova/network/linux_net.py

@@ -804,6 +804,7 @@ def clean_conntrack(fixed_ip):
         LOG.exception(_('Error deleting conntrack entries for %s'), fixed_ip)
 
 
+@utils.synchronized('lock_gateway', external=True)
 def initialize_gateway_device(dev, network_ref):
     if not network_ref:
         return