zebra: copy logical-router-command under vrf subnode

zebra: copy logical-router-command under vrf subnode

a vty command is added:
in addition to this command ( kept for future usage):
- [no] logical-router-id <ID> netns <NETNSNAME>
a new command is being placed under vrf subnode
- vrf <NAME>
   [no] netns <NETNSNAME>
  exit

This command permits to map a VRF with a Netnamespace.
The commit only handles the relationship between vrf and ns structures.
It adds 2 attributes to vrf structure:
- one defines the kind of vrf ( mapped under netns or vrf from kernel)
- the other is the opaque pointer to ns
The show running-config is handled by zebra daemon.

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