[Ovmsdev] Reducing wifi/netmanager log messages

[Stephen Casner]

When driving, and therefore out of range of the home wifi, the
following messges emitted every 10 seconds will fill up the log:

I (2019-01-04 03:46:36.839) netmanager: Interface priority is pp2 (10.170.41.247/255.255.255.255 gateway 10.64.64.64)
I (2019-01-04 03:46:36.889) netmanager: Set DNS#0 8.8.8.8
I (2019-01-04 03:46:36.939) netmanager: Set DNS#1 8.8.4.4
I (2019-01-04 03:46:36.989) esp32wifi: STA disconnected with reason 201
I (2019-01-04 03:46:46.839) netmanager: Interface priority is pp2 (10.170.41.247/255.255.255.255 gateway 10.64.64.64)
I (2019-01-04 03:46:46.899) netmanager: Set DNS#0 8.8.8.8
I (2019-01-04 03:46:46.949) netmanager: Set DNS#1 8.8.4.4
I (2019-01-04 03:46:46.999) esp32wifi: STA disconnected with reason 201
I (2019-01-04 03:46:56.839) netmanager: Interface priority is pp2 (10.170.41.247/255.255.255.255 gateway 10.64.64.64)
I (2019-01-04 03:46:56.889) netmanager: Set DNS#0 8.8.8.8
I (2019-01-04 03:46:56.939) netmanager: Set DNS#1 8.8.4.4
I (2019-01-04 03:46:56.979) esp32wifi: STA disconnected with reason 201

I'd like to add code to suppress those messages when there is no
change in state.  Any objections?

                                                        -- Steve