Fix importing security group tests

Importing StatelessSecGroupDualStackDHCPv6StatelessTest was causing
a type error in case CONF.neutron_plugin_options.firewall_driver
was set to 'openvswitch' [1]. This patch fixes the issue by
separation code to a class with base functions, that is never skipped
and classes with tests that optionally can be skipped.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=2249407

Change-Id: I90b9cf495f35aca7a9171dc685696ae4cdd4d0f2
1 file changed