Avoid testing code duplication which introduced testing bugs

SecurityGroupAgentEnhancedRpcTestCase duplicated code in
SecurityGroupAgentRpcTestCase setUp, also
TestSecurityGroupAgentEnhancedRpcWithIptables duplicated
code from TestSecurityGroupAgentWithIptables setUp()
introducing bugs by improper initialization, like a missing
    self.iptables.use_ipv6 = True
which in combination with tests.unit.test_ipv6.TestIsEnabled
produced inconsistent testing results.

Change-Id: Ic0b596b6fe1d8273df197e9426abad818832c00a
Closes-bug: #1365829
Stop tracking connections in DVR FIP Namespace

For DVR, connections to external network using floating IPs do not
need to be tracked in FIP namespace because they are being
already tracked in router namespace.
This fix adds iptable rules to not track connections.

Change-Id: I378039e311763d114860cb70ded7349d6580855a
Closes-bug: #1360395
Merge "Avoid testing code duplication which introduced testing bugs"
Merge "Stop tracking connections in DVR FIP Namespace"