Reclass adapter improvements

These three commits improve the reclass adapters in backwards-compatible ways.

1. Logging messages are amended;

2. It is not possible to specify the path to a reclass source checkout in the Salt master configuration file, if reclass isn't properly installed;

3. reclass is made to default to looking for its inventory in the first 'file_roots' or class 'base' (this isn't yet working due to Salt issue #5951).