tools: Rename engine_ca to cert_file.

Renamed engine_ca to cert_file. This patch fixes API object creation by
renaming engine_ca.

Change-Id: Ia1b7855acfea814c2a66a1869247d580c6d879cb
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=894690
Signed-off-by: Kiril Nesenko <knesenko@redhat.com>