This is a follow-up to 2216d3ca8d, which
implemented the StartInterval for health-checks, but did not add validation
for the minimum accepted interval;
> The time to wait between checks in nanoseconds during the start period.
> It should be 0 or at least 1000000 (1 ms). 0 means inherit.
This patch adds validation for the minimum accepted interval (1ms).
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>