When parsing a NMSettingTeam/NMSettingTeamPort from GVariant, the JSON
"config" is always preferred. If that field is present, all other
attributes are ignored (aside from validating that the individual fields
are as expected).

The idea is that the JSON config anyway must contain everything that artificial
properties could. In this mode, also no validation is performed and the setting is
always accepted (regardless whether libnm thinks the setting verifies).

When a libnm client created the setting via the artificial properties,
only send them via D-Bus and exclude the JSON config. This turns on
strict validation server side.

Now we actually get validation of the settings:

  $ nmcli connection add type team team.runner-tx-hash l3
  Error: Failed to add 'team' connection: team.runner: runner-tx-hash is only allowed for runners loadbalance,lacp

this is obviously a change in behavior as previously all settings
would have been accepted, regardless whether they made sense.