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: I4e7f4f071aded4083d5db23bf2ad27c6433f60cd
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=894688
Signed-off-by: Kiril Nesenko <knesenko@redhat.com>