lib: externalise vrf and ns creation

lib: externalise vrf and ns creation

In addition to have the possibility to create from vty vrf based on a
netns backend, the API will be made accessible from external, especially
for zebra that will handle the netns discovery part. This commit is
externalising following functions:
- netns_pathname
- ns_handler_create
- vrf_handler_create

Also, the VRF initialisation case when under NETNS backend is changed,
since the NS identifier may not be known at the configuration time,but
may be known later, under discovery process.

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