Mock check if ipv6 is enabled in L3 agent unit tests
Mock check if ipv6 is enabled in L3 agent unit tests

Without this mock UT related configure_ipv6 method were failing
e.g. on systems when IPv6 was disabled or on systems where such
check should be done differently, like MacOS.

Change-Id: I6d13ab1db1d5465b2ff6abf6499e0d17e1ee8bbb

mahak/neutron,mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron,openstack/neutron