cve-update-nvd2-native: make number of fetch attemtps configurable

cve-update-nvd2-native: make number of fetch attemtps configurable

Sometimes NVD servers are unstable and return too many errors.

Last time we increased number of attempts from 3 to 5, but
further increasing is not reasonable as in normal case
too many retries is just abusive.

Keep retries low as default and allow to increase as needed.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>