cve-update-nvd2-native: faster requests with API keys

cve-update-nvd2-native: faster requests with API keys

As per NVD, the public rate limit is 5 requests in 30s (6s delay).
Using an API key increases the limit to 50 requests in 30s (0.6s delay).
However, NVD still recommends sleeping for several seconds so that the
other legitimate requests are serviced without denial or interruption.
Keeping the default sleep at 6 seconds and 2 seconds with an API key.

For failures, the wait time is unchanged (6 seconds).

Reference: https://nvd.nist.gov/developers/start-here#RateLimits

Signed-off-by: Dhairya Nagodra <dnagodra@cisco.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 5c32e2941d1dc3d04a799a1b7cbd275c1ccc9e79)
Signed-off-by: Steve Sakoman <steve@sakoman.com>