zebra: keep one command to handle vty with/without table param

zebra: keep one command to handle vty with/without table param

That fix is a workaround from a vtysh limitation.
Because table identifier should be accessible in configuration only for
vrf netns backends, there was a need to differentiate the vty commands.
Unfortunately, vtysh parses the two commands without knowing which
command has really been installed.
Using one single vty command will avoid having this issue in vtysh.

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