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: Ibdfae742e8dfea58eb19b61987e347003b69b57b
Bug-Url: https://bugzilla.redhat.com/1032689
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>