addons: vrf: be more friendly to older iproute2 versions

addons: vrf: be more friendly to older iproute2 versions

Ticket: CM-12049
Reviewed By: dsa, julien, nikhil
Testing Done: tested with and without /etc/iproute2/rt_tables.d/

OS does not have /etc/iproute2/rt_tables.d, on errors just info:
$ifreload -a -v
info: unable to save iproute2 vrf to table map ([Errno 2] No such file
or directory: '/etc/iproute2/rt_tables.d/ifupdown2_vrf_map.conf')

info: cannot find /etc/iproute2/rt_tables.d. pls check if your iproute2
version supports rt_tables.d

OS has /etc/iproute2/rt_tables.d, on errors warn:
$ifreload -a
warn: unable to save iproute2 vrf to table map ([Errno 2] No such file

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>