Check conntrack rule cleanup on floating IP disassociate

Check conntrack rule cleanup on floating IP disassociate

Check that conntrack rule is cleaned up immediately after floating IP is
disassociated.

Extend NetcatTester class to allow to specify different addresses for
client and server sides of connection. Fixed the class not to pass -k
option for UDP connections (otherwise the test fails when used with nmap
netcat implementation).

Change-Id: Iaa11e65a0bcf7860d4caa629c3107e5a0a48fd39