Change the way we set VLANs very slightly. We used to use the vmVlan

OID from the CISCO-VLAN-MEMBERSHIP MIB. But, this MIB does not seem
to be supported by all Cisco switches. So, we now use vlanPortVlan
from the CISCO-STACK MIB.

So, we no longer need CISCO-VLAN-MEMBERSHIP-MIB at all, which should make
snmpit usable on a wider variety of Ciscos.