service: load maker, model and stream from config

For the longest time, we used stub values for the maker, model and stream
properties. With the recent drive to complete the online side of SystemOTA
we are now able to load the values from the configuration system at
startup.

Tests are adjusted to use the test-* prefix for the values, in order to
avoid missing stub values anywhere. Integration tests are modified to
measure the values which are now set in the prepare logic of the test
suite.

There's a missing bit of integration between the writable update stream
property and the configuration system. At the moment changes to that
property are lost.

The update server is not used yet. It is also not exposed in the API,
on purpose, so that it cannot be modified or even read before the
interface between the client and the server is implemented.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>