config: added backward compatibilty

Added backward compatibility supporting upgrade from
3.1 version which used the keyword engine-ca instead
of cert-file.

Change-Id: I43befe3fa63e1fb5c624fd528399e867c2b5e882
Bug-Url: https://bugzilla.redhat.com/1046309
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>