Avoid testing code duplication which introduced testing bugs

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