Be able to enumerate device attribute pollers

Often an OID will return an integer value which is meanless without a lookup to get an additional value that is useful to humans.

Therefore it would be great when creating a device attribute poller to be able to define an optional enumerated list for the return value to be match against.

Then when a user is looking at device attributes on a host page they would see the looked up value for the attribute and not the raw return from the OID.