addons: vrf: redo iproute2 vrf interface map handling

addons: vrf: redo iproute2 vrf interface map handling

Ticket: CM-10188, CM-10061
Reviewed By: dsa, nikhil, julien
Testing Done: Tested static routes with vrf names for tables

This patch does the following:
- if a single vrf device is present in the config,
builds the vrf map by reading vrf interfaces from the kernel (with
existing link cache. Builds a shadow vrf only attribute cache)
- reads existing table map and adjusts it if required
- main change is the iproute2 map file on disk is updated
immediately on vrf creation, so that static routes used along with the
vrf slaves can use the vrf name for the table. This also helps dhclient dns
hook script which may use mgmt table name directly.
- cleans up default routes on down

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