Fix snmp overriding of auto-configured table fields (#4208)

Whenever the snmp plugin was configured with a table with automatic field
discovery, if one of those fields was explicitly overridden in the config and
the value of is_tag was changed, the field would be duplicated, once as a tag
& once as a field.

This change fixes the behavior so that if a field is explicitly configured,
automatic table field discovery doesn't touch it.