bgpd, lib, zebra: Add ability to retrieve ifp without specifying a vrf

bgpd, lib, zebra: Add ability to retrieve ifp without specifying a vrf

There are cases where we get an interface name but do not have a
corresponding vrf.  We care about getting an interface pointer
so just provide a function that searches all vrf's for the ifp.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Radhicak Mahankali <radhika@cumulusnetworks.com>
Reviewed-by: Vivek Venkatraman <vivek@cumulusnetworks.com>