mon: `config get <who>` does not allow `who` as 'mon'/'osd'
`config set mon` is allowed, but `config get mon` is not.
This is due to <who> on `get` being parsed as an EntityName, which does not understand what 'mon' is, but will understand what 'mon.' or 'mon.a' is.
`config get` needs to understand values of `who` that do not include the dot, because adding the dot makes it for a different entity than what we have on the map.