Bug 6992 - legacy: ignore IPv6 router interface

Bug 6992 - legacy: ignore IPv6 router interface

Since NeutronL3Adapter doesn't know ipv6, it should ignore
ipv6 router interface. Especially floating ip doesn't work
depending on the situation.

Neutron Router can be attached with both of IPv6 only port and IPv4 only
port. IPv6 only port should be ignored when programming for floatingip.
Otherwise networkIdToRouterMacCache can populated with mac address of
IPv6 only port, and programFlowsForNeutronRouterInterface() may program
flow with IPv4 address with mac address of IPv6 port. It should use
mac address of the IPv4 port. Otherwise floatingip is broken.

Change-Id: I87ec4e2537666f5db6826ca93be8be1793157ec7
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>