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: Ie4b06eea10a31b675fac0cae98465988dd66894d
Bug-Url: https://bugzilla.redhat.com/1055504
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>