Port from snappy branch: no exception on getprop

[Barry Warsaw]

The snappy branch of si removed the exception logging if getprop is not available. This can probably be handled as a configuration file change, now that si supports config.d and multiple config files. At least, add a test for this to be so.

[Barry Warsaw (barry)]

There should be nothing to do for si 3.0. the [hooks]device hook is only called if the configuration files don't have a [service]device variable, which I believe snappy devices will always have. So it should never call the default systemimage.device.SystemProperty.get_device() method, and thus never call getprop.

I'll mark this as invalid, but please re-open if I'm mistaken.