zebra: add the registration mechanism for netns

zebra: add the registration mechanism for netns

If vrf backend is netns, then the zebra will create its own
zebra_ns context for each new netns discovered. As consequence,
a routing table, and other contexts will be created for each
new namespace discovered. When it is enabled, a populate process
will be done, consisting in learning new interfaces and routes, and
addresses from other NETNS.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>