Avoid re-adding iptables rules for instances that have disappeared

The remove_filters_for_instance() method fails silently if the
instance's chain is gone (i.e. it's been deleted). If this
happens while we're refreshing security group rules, we will
not notice this case and re-add stale rules for an old instance,
breaking our firewall for new instances.

This adds a quick check after we've captured the lock to see if
the associated chain exists, and bails if it doesn't.

Change-Id: Ic75988939f82de49735d85fe99a9eecd4baf45c9
Related-bug: #1182131
Merge "Avoid re-adding iptables rules for instances that have disappeared"