ifName `agent` default value

## Feature Request

Opening a feature request kicks off a discussion.
As discussed in https://github.com/influxdata/telegraf/pull/7763#issuecomment-676801651

### Proposal:

Let the default value of these be the same:

* `agent` for `processors.ifname`
* `agent_host` for `inputs.snmp`
* `agent_address` for `inputs.sflow`

### Current behavior:

The default value (agent) mostly never useful.

### Desired behavior:

The default value should be a logical one that makes sense or most users would not need to change.

- [ ] #14111
- [ ] #14112
- [ ] #14117
- [ ] #14899

Start by reading the processors.ifname, inputs.snmp, and inputs.sflow plugin entry points and the discussion linked in the issue. Compare their current default values and review the related issues #14111, #14112, #14117, and #14899. Done means the defaults are logically aligned and the relevant plugin tests or configuration expectations pass.

Written by the indexing model from the issue text.