zebra: extend table manager per vrf, add vty configuration

zebra: extend table manager per vrf, add vty configuration

Because vrf backend may be based on namespaces, each vrf can
use in the [16-(2^32-1)] range table identifier for daemons that
request it. Extend the table manager to be hosted by vrf.

That possibility is disabled in the case the vrf backend is vrflite.
In that case, all vrf context use the same table manager instance.

Add a configuration command to be able to configure the wished
range of tables to use. This is a solution that permits to give
chunks to bgp daemon when it works with bgp flowspec entries and
wants to use specific iptables that do not override vrf tables.

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