In some cases we want the returned value to be stripped. In some cases,
we want to read the raw value instead of the string parsed by GKeyFile.
Add an flags argument to nm_config_data_get_value(). It is up to the caller
to determine the exact meaning (and whether to strip).

By adding the flags argument, the caller can get the desired behavior easier
without having to workaround it afterwards. But more importantly, it becomes
apparent that there are different ways to retrieve the value and the caller
should decide on the details.