Add firewall blink + remote SG functional tests

Add firewall blink + remote SG functional tests

This tests that firewall still does its purpose even when rules are
being updated. That means there is no short period of time where
security groups are inactive during update.

Part of this patch introduces Pinger class. This object provides
capability of sending ICMP packets asynchronously and after
it's stopped it provides statistics like how many packets were
sent and how many were received. Note the difference between
assert_ping() functions, which are synchronous.

Another testing of remote security groups is also added.

Related-bug: #1461000
Change-Id: I6251ee264396f8dbc9b284758b96e5cdc6ac500b