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
Merge "Avoid testing code duplication which introduced testing bugs"