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: I8552f56c4e70be5235ce148c0677ebaed2065df9
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=894689
Signed-off-by: Kiril Nesenko <knesenko@redhat.com>