[KeyManager] loadCertificateFromFile implementation

[KeyManager] loadCertificateFromFile implementation

[Verification]
var cert = new tizen.Certificate("cert_file1", null, true);
cert.loadFromFile("/opt/usr/media/Others/cert.cer",
    function() {
        console.log('ok');
    },
    function(e) {
        console.log(e.code);
        console.log(e.message);
    }
);

Change-Id: I81380b69a835ef0e3128c92b8c541bec3538f7ee
Signed-off-by: Przemyslaw Ciezkowski <p.ciezkowski@samsung.com>