Change keyToFilename/readSSconfFile interaction

Currently, keyToFilename uses itself the default config path. In the
light of making that an function in the IO monad, let's remove the
"default" path functionality from this function and make it always
require the config path; its caller, readSSConfFile, can then do the
maybe/default path change.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>