config: avoid to change existing configuration files

When reading configurarion files, just warn the user
that there is a deprecated key and warn the user
to fix it, avoiding to change existing configuration
files.
(cherry picked with modifications from
501e97de32a51f5d45db2582bc0bed691d4bf374)

Change-Id: I55c707424524826235f1faba52c0fafd72a27300
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>