doc: add information about dynamic update of default vrf name

doc: add information about dynamic update of default vrf name

It is possible to dynamically change default VRF name, if vrf backend is
a netns backend. By creating a link to the default netns in
/var/run/netns folder, then the file name will be used to name the
default VRF. If no backend netns is chosen, it is explained that it is
still possible to statically configure the default vrf name to new
define.

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