bgpd: add utility to check if a vrf is active

bgpd: add utility to check if a vrf is active

From RFC4382:
A VRF is
up(1) when there is at least one interface associated
with the VRF whose ifOperStatus is up(1).  A VRF is
down(2) when:
a. There does not exist at least one interface whose
   ifOperStatus is up(1).
b. There are no interfaces associated with the VRF.

Run through interfaces associated with a vrf and return
true if there is one in the up state.

Signed-off-by: Pat Ruddy <pat@voltanet.io>